feat(match): match any AbstractVector via Indexable[...] and abstract-array heads

[Roger-luo]

Closes #36.

Motivation

@match view([1, 2, 3], :) begin
    [1, x, 3] => x
    _ => nothing
end

returned nothing because [1, x, 3] checks isa Vector, but view(...) produces a SubArray. As discussed in #36, bare [...] should stay Vector-only (an array literal builds a Vector, so the pattern deconstructs a Vector). This PR adds explicit syntax for matching any AbstractVector instead of widening [...].

What's new

Pattern	Matches
[1, x, 3]	Vector only (unchanged)
Int[1, x, 3]	Vector{Int} — concrete head still means element type (unchanged)
Indexable[1, x, 3]	any AbstractVector (reserved keyword)
AbstractVector[1, x, 3] / any abstract-array head	isa that type

@match view([1, 2, 3], :) begin
    Indexable[1, x, 3] => x   # ⇒ 2   (also: AbstractVector[1, x, 3])
    _ => nothing
end

Design notes

• Indexable is a scanner-level reserved keyword → Pattern.Indexable, matching AbstractVector. No import needed, can't be shadowed — that's its value over spelling out AbstractVector.
• A Ref head T[...] is a container constraint only when T is an abstract array type (T <: AbstractArray && isabstracttype(unwrap_unionall(T))). Concrete heads (Int, Any, Union{...}, Vector, SubArray, ...) keep their existing element-type meaning, so nothing existing breaks.
• Zero runtime cost. is_container_head is Base.@assume_effects :foldable; the is_container_head(T) ? T : Vector{T} branch const-folds away (verified via @code_typed).
• firstindex-relative indexing. The shared collection-deconstruction machinery now indexes off firstindex/lastindex instead of a hard-coded 1, so non-1-based vectors (e.g. OffsetArray) deconstruct correctly. For Vector/Tuple, firstindex is 1 and folds away — no perf regression (perf-regression testset still passes).

Tests

• Scanner: Indexable[...] → Pattern.Indexable, plus show round-trip.
• Behavioral (test/match/examples/basic.jl): the exact should vector pattern match any AbstractVector? #36 case, Indexable/AbstractVector on views/ranges/plain vectors, bare [...] still rejecting a SubArray, typed splats under an abstract container head, and a minimal non-1-based OffsetVec <: AbstractVector across leading/trailing/interior splat shapes.

Full suite passes (data 325, match 140, derive 30, perf 5). Files are blue-formatted. Project.toml/CHANGELOG.md left untouched per release-please conventions.

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Project	Deployment	Actions	Updated (UTC)
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[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.91%. Comparing base (901dcbc) to head (f9c0cc1).

Files with missing lines	Patch %	Lines
src/match/emit/collection/vect.jl	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
+ Coverage   91.86%   91.91%   +0.05%     
==========================================
  Files          43       43              
  Lines        1708     1732      +24     
==========================================
+ Hits         1569     1592      +23     
- Misses        139      140       +1     

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Benchmark Results (Julia v1)

Time benchmarks

	main	f9c0cc1...	main / f9c0cc1...
adt_transform/transform n=100	6.27 ± 0.23 μs	6.2 ± 0.24 μs	1.01 ± 0.054
adt_transform/transform n=1000	0.0589 ± 0.00098 ms	0.0584 ± 0.001 ms	1.01 ± 0.024
linked_list/sum n=100	0.511 ± 0.001 μs	0.511 ± 0.001 μs	1 ± 0.0028
linked_list/sum n=1000	5.6 ± 0.039 μs	5.58 ± 0.04 μs	1 ± 0.01
time_to_load	0.0816 ± 0.00064 s	0.081 ± 0.00012 s	1.01 ± 0.0081

Memory benchmarks

	main	f9c0cc1...	main / f9c0cc1...
adt_transform/transform n=100	0.204 k allocs: 6.34 kB	0.204 k allocs: 6.34 kB	1
adt_transform/transform n=1000	2.02 k allocs: 0.0619 MB	2.02 k allocs: 0.0619 MB	1
linked_list/sum n=100	0 allocs: 0 B	0 allocs: 0 B
linked_list/sum n=1000	0 allocs: 0 B	0 allocs: 0 B
time_to_load	0.145 k allocs: 11 kB	0.145 k allocs: 11 kB	1