fix: Correct three defects in how a module is staged

[tablackburn]

Summary

Fixes three defects in Build-PSBuildModule's staging logic, sequenced #211 → #210 → #212
because each is only visible once the one before it is fixed. Closes #210, #211, #212.

• Staging flattens a culture directory's .psd1 into the output root #211 — the loose-file glob used -Path $Path with -Include and -Depth 1, so it
  recursed one level and copied en-US/Messages.psd1 flat into the output root. Both
  halves of the fix are required: $Path\* alone still matches the culture directory, and
  dropping -Depth alone matches nothing, because without recursion -Include filters
  against the leaf of -Path. The same pattern in the readme discovery in psakeFile.ps1
  and IB.tasks.ps1 is fixed with it.
• Compiling drops a hand-written about_<Module>.help.txt unless CopyDirectories names the culture directory #210 — compile mode staged the root files and CopyDirectories and nothing else, so a
  hand-written about topic reached the output only if CopyDirectories happened to name the
  culture directory. Compile mode now stages a source culture directory on its own.
• Compile and non-compile modes disagree on whether the readme or a source about file wins #212 — the two modes disagreed about whether the source about topic or the readme won,
  by accident of statement ordering. Source wins in both now, with a warning:
  ConvertReadMeToAboutHelp performs no conversion, so "readme wins" replaces conformant
  help with raw Markdown.

Host-dependence, measured

-Depth with -Include degrades to a full -Recurse on Windows PowerShell 5.1. Same glob,
same tree (tests/), two hosts:

Host	before	after
PowerShell 7.6.5	root + depth 1	root only
Windows PowerShell 5.1	11 matches, down to depth 3	root only (2)

Readme discovery, on 5.1: before, tests/TestModule/README.md came back first and
Select-Object -First 1 would have taken it. After: the project root's readme, on both hosts.

Staging, before and after

Fixture built with CopyDirectories = @(), a readme, a hand-written en-US about topic, and
a bin/ holding no help (bin is the culture name for Bini).

	before / compile	after / compile	before / non-compile	after / non-compile
stray root Messages.psd1	present	gone	present	gone
en-US/Messages.psd1	dropped	staged	staged	staged
about topic content	readme	hand-written	hand-written	hand-written
warning when readme skipped	—	yes	none	yes
bin/ staged	no	no	n/a (bulk copy)	n/a

Closing the blind spot

tests/fixtures/PSBuildTestFixture had no culture directory, no .psd1 below its root and
no about topic, so the suite was structurally incapable of seeing any of these three — which
is how all of them survived since 2018. It now ships en-US/Messages.psd1 and a hand-written
en-US/about_PSBuildTestFixture.help.txt.

Assertions that moved because of it, each deliberate:

• Build-PSBuildModule.tests.ps1 — "Produces only the manifest and a monolithic root module"
  keeps its count of 2. That count is now the Staging flattens a culture directory's .psd1 into the output root #211 guard: the flattened Messages.psd1 made
  it 3.
• The two readme-conversion contexts (including the Build-PSBuildModule silently writes no about help file when the culture directory already exists #207 regression test) now delete the
  fixture's about topic first, so they still cover conversion rather than Compile and non-compile modes disagree on whether the readme or a source about file wins #212's precedence.
  Build-PSBuildModule silently writes no about help file when the culture directory already exists #207's guard was on the culture directory; Compile and non-compile modes disagree on whether the readme or a source about file wins #212's is on the about file, so the empty
  destination directory Build-PSBuildModule silently writes no about help file when the culture directory already exists #207 is about still behaves as it did.
• Build-PSBuildHelp.tests.ps1 — the cabinet context asserted "no warnings at all". PlatyPS
  1.x warns File '' is not a valid help file type for any file in the module's locale
  directory that is not the generated MAML — an about topic included, with an empty name in
  its own message. Upstream behavior about module content, so the assertion narrowed to "no
  landing-page warning", which is the defect it pins.

Test plan

• ./build.ps1 -Task Test green: 608 passed, 0 failed, 3 skipped (591 before).
• 17 new tests, covering both modes: culture staging without CopyDirectories, no flattened
  root .psd1, bin/ not staged, readme conversion in both modes, and source-about-topic
  precedence with the warning in both modes.
• The Build-PSBuildModule silently writes no about help file when the culture directory already exists #207 regression context passes unchanged in substance.
• Changelog entry per issue, plus migration entries for Staging flattens a culture directory's .psd1 into the output root #211 and Compile and non-compile modes disagree on whether the readme or a source about file wins #212 — the two changes a
  consumer sees on upgrade.

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Test Results

    4 files  ± 0    909 suites  +20   2m 9s ⏱️ -18s
  613 tests +17    610 ✅ +17   3 💤 ±0  0 ❌ ±0 
2 435 runs  +68  2 363 ✅ +68  72 💤 ±0  0 ❌ ±0 

Results for commit f97dac2. ± Comparison against base commit 3ea9e82.

This pull request removes 2 and adds 19 tests. Note that renamed tests count towards both.

Build-PSBuildModule.Converting the readme into about help.Writes the readme as the about help file in the culture directory
Build-PSBuildModule.Converting the readme into about help.Writes the readme content unchanged

Build-PSBuildModule.Building with compilation.Does not flatten a culture directory file into the output root
Build-PSBuildModule.Building with compilation.Does not stage a directory that only shares a name with a culture
Build-PSBuildModule.Building with compilation.Stages Messages.psd1 in the culture directory without CopyDirectories naming it
Build-PSBuildModule.Building with compilation.Stages about_PSBuildTestFixture.help.txt in the culture directory without CopyDirectories naming it
Build-PSBuildModule.Building without compilation.Does not flatten a culture directory file into the output root
Build-PSBuildModule.Building without compilation.Stages Messages.psd1 in the culture directory
Build-PSBuildModule.Building without compilation.Stages about_PSBuildTestFixture.help.txt in the culture directory
Build-PSBuildModule.Choosing the about help culture.Leaves the hand-written topic in its own culture directory alone
Build-PSBuildModule.Converting the readme into about help when compiling.Emits no warning
Build-PSBuildModule.Converting the readme into about help when compiling.Keeps the rest of the source culture directory
…