[chiliec]

What

Fixes #965: decode time is quadratic in the number of CustomUnmarshaler nodes.

Why

newFormatter walks the entire document token chain to rebuild its
tokenToOriginMap on every call. decodeByUnmarshaler creates a fresh formatter
(via unmarshalableDocument) for each CustomUnmarshaler/BytesUnmarshaler node,
so decoding N such nodes rebuilds the whole-document map N times → O(N²).

The map depends only on the document's token chain, which is the same for every
node in that document, so it can be built once and reused.

Change

• Extract the map construction into format.NewTokenToOriginMap and let
  newFormatter accept a prebuilt map (new FormatNodeWithTokenOriginMap;
  FormatNodeWithResolvedAlias still works, passing nil).
• The decoder builds the map once per document (keyed on the token-chain head so a
  reused Decoder across different trees rebuilds correctly) and reuses it for all
  custom-unmarshaler formatting.

Formatter output is unchanged — only the per-node rebuild is removed.

Tests

Added BenchmarkUnmarshalWithCustomUnmarshaler mirroring the issue's repro.

Validation (go1.23.4, go test ./... all pass):

Before (ns per line stays proportional to line count — quadratic):

n=250    101763 ns/n
n=500    199369 ns/n
n=1000   441802 ns/n
n=2000  1100730 ns/n

After (flat — linear):

n=250   ~4465 ns/n
n=500   ~4467 ns/n
n=1000  ~6794 ns/n
n=2000 ~11678 ns/n

At n=2000 the decode drops from ~2.2s to ~23ms. Full suite green on the branch.

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Codecov Report

❌ Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.65%. Comparing base (edee2f9) to head (ebb7671).
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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #966   +/-   ##
=======================================
  Coverage   80.65%   80.65%           
=======================================
  Files          22       22           
  Lines        6845     6856   +11     
=======================================
+ Hits         5521     5530    +9     
- Misses        905      906    +1     
- Partials      419      420    +1     

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[orivej]

@chiliec Thank you! Please review #967, I think it is simpler.

[chiliec]

Closing in favour of #967, which removes the full-document scan entirely and is faster.