Preserve structure in docgen function details

[cristianoc]

Part of the n-ary functions series tracked in #8557 (item 10 of 10, the series finale; stacked on item 9's branch via base branch; each PR merges independently).

What

Function labels in Outcometree were strings, with optionality encoded by a leading question mark. That forced printers to decode the spelling and left downstream consumers without the label structure already known by the type system. Store Noloc.arg_label directly on Otyp_arrow, update both printers to match it exhaustively, and remove the unproduced Octy_arrow constructor.

The doc generator previously walked Types.type_expr independently and flattened every reachable constructor into one list. Nested arrows became outer parameters, tuples and type variables disappeared, labels and optionality were lost, and non-function values acquired fabricated zero-parameter signatures. Build details from the normalized Outcometree instead: parameters retain their metadata, constructors, variables, tuples, and functions form recursive nodes, uncommon forms remain visible through a rendered fallback, and only top-level arrows receive signature details.

Update the published RescriptTools.Docgen types and snapshots for the intentionally breaking JSON shape, and correct the implementation's stale alias tag to match the signature tag declared by its interface. The documentation site drops value details before publishing its data, but third-party consumers of rescript-tools doc need the changelog warning.

Focused fixtures cover labeled and optional parameters, generic variables, callbacks, tuple returns, returned functions, fallback rendering, and non-function values. Compiler, tools, analysis, syntax, roundtrip, and full test suites remain green.

Signed-off-by: Cristiano Calcagno cristianoc@users.noreply.github.com

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[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.95%. Comparing base (403b094) to head (17d8b22).

Files with missing lines	Patch %	Lines
compiler/ml/oprint.ml	0.00%	4 Missing ⚠️
tools/src/tools.ml	95.45%	2 Missing ⚠️

Additional details and impacted files

@@                       Coverage Diff                        @@
##           codex/nary-value-constraints    #8576      +/-   ##
================================================================
+ Coverage                         75.93%   75.95%   +0.01%     
================================================================
  Files                               474      474              
  Lines                             62902    62905       +3     
================================================================
+ Hits                              47765    47779      +14     
+ Misses                            15137    15126      -11     

Files with missing lines	Coverage Δ
compiler/ml/printtyp.ml	64.32% <100.00%> (-0.22%)	⬇️
compiler/syntax/src/res_outcome_printer.ml	58.49% <100.00%> (-0.09%)	⬇️
tools/src/tools.ml	80.17% <95.45%> (+2.70%)	⬆️
compiler/ml/oprint.ml	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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commit: 17d8b22