fix(index): keep node line index sub-linear for single-line JSON documents

[s04]

Fixes #621

Problem

Since v0.38.0 (4a22282) the per-line node index stores entries in a slice that is scanned linearly on every insert and lookup. For YAML that is cheap, but a minified JSON document puts every node on line 1, so building the index is O(n²) in node count. An 8 MB single-line spec (~700k nodes) pins a core inside addNodeLineEntry for minutes; v0.37.3 builds it in ~1.5 s.

Change

index/map_index_nodes.go:

• addNodeLineEntry is now append-only. No scan per insert.
• New sortNodeLines runs once at the end of MapNodes: stable-sorts each line by column and collapses duplicate columns keeping the last write. mapNodesRecursive writes parents after children, so parents still win collisions, matching the previous behaviour and the original map semantics.
• lookupNodeLines is a binary search over the sorted line.

The [][]nodeLineEntry type and all call sites (GetNode, GetNodeMap, FindNodeOrigin, the KeyNode lookup in extract_refs_lookup.go) are unchanged. One behavioural nuance: entries[0] on a line is now the leftmost node rather than the first one visited. For the KeyNode lookup that is the intended node, and in practice they coincide because DFS visits the leftmost child first.

Tests

• TestSpecIndex_MapNodes_SingleLineDocument: minified document with ~16k nodes on one line; every node is retrievable by position and the legacy map agrees with the line index.
• TestSpecIndex_SortNodeLines_OrdersAndDedupes: out-of-order inserts and duplicate columns.
• BenchmarkSpecIndex_MapNodes_SingleLine: ~200k nodes on one line.

BenchmarkSpecIndex_MapNodes_SingleLine   before (origin/main): 101,835,604,666 ns/op
                                         after:                    301,491,542 ns/op
BenchmarkSpecIndex_MapNodes              unchanged (~378 µs/op)

Full go test ./... passes. The 8 MB real-world single-line spec builds its V3 model in ~0.7 s with this change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.78%. Comparing base (ca6ca73) to head (d0be530).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #622   +/-   ##
=======================================
  Coverage   99.78%   99.78%           
=======================================
  Files         283      283           
  Lines       34456    34468   +12     
=======================================
+ Hits        34382    34394   +12     
  Misses         46       46           
  Partials       28       28           

Flag	Coverage Δ
unittests	99.78% <100.00%> (+<0.01%)	⬆️

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