Add content-based monotype interning

[lukewilliamboswell]

Summary

Replaces the append-only monotype store with content-based interning, ensuring structurally identical types get the same TypeId. This eliminates ~200 lines of structural equality workarounds and fixes downstream cache inconsistencies caused by duplicate monotype indices.

Key changes

• Content-based interning: internFunc, internRecord, internTagUnion, etc. deduplicate by structural content using per-kind AutoHashMapUnmanaged intern maps
• TypeId + resolve() API: Replace getMonotype() tagged-union pattern matching with resolve() + typed accessors (funcArgs, recordFields, tupleElems, etc.)
• Structural name pool: Tag/field names use StructuralNameId from a shared pool instead of per-module Ident.Idx, eliminating cross-module ident remapping
• Nominal instance cache: Deduplicates nominal type instantiations by (origin, type_name, args), replacing the old nominal_cycle_breakers map
• O(1) list interning: Replace O(n) linear scans in internIdxList/internFieldList/internTagList with fingerprint-keyed hash maps with overflow chaining
• Cross-module cleanup: remapMonotypeBetweenModules simplified to just .rec placeholder resolution (documented with regression tests)
• Bug fixes: Double-resolve caching, tag comparison via identMatchesText() instead of fragile null-guard pattern

Files changed

File	What
src/mir/Monotype.zig	Core interning rewrite: TypeId, resolve(), intern maps, name pool, list hash maps
src/mir/Lower.zig	Adapt to TypeId API, simplify cross-module remapping, nominal cache
src/lir/MirToLir.zig	Adapt to resolve() + typed accessors, cache double-resolves
src/layout/mir_monotype_resolver.zig	Adapt to new TypeId representation
src/mir/MIR.zig	Store type alias update
src/mir/LambdaSet.zig	Adapt to TypeId API
src/cli/main.zig	Deinit signature update
src/eval/dev_evaluator.zig	Deinit signature update
src/layout/store_test.zig	Test adaptation
src/mir/test/lower_test.zig	Test adaptation to new intern APIs
src/check/test/cross_module_mono_test.zig	New cross-module recursive nominal tests
test/fx/cross_module_recursive_nominal.roc	New test fixture

Test plan

• zig build test-eval --summary all — 1227/1227 passed
• Full CI

[lukewilliamboswell]

From what I can tell this CI failure is unrelated... I'm not sure what has caused it recently.

Running command: zig build -Dfuzz -Dsystem-afl=false -Doptimize=ReleaseFast -Dcpu=x86_64_v3 
/Users/runner/.cache/zig/p/AFLplusplus-4.21.0-aA1y4UtxAABpnSIF7ARSYDMRyqNcI-2Rwa5UeSsuw70v/build.zig:27:20: error: root source file struct 'std' has no member named 'BoundedArray'
    var flags = std.BoundedArray([]const u8, 16){};
                ~~~^~~~~~~~~~~~~
/Users/runner/work/_temp/34017541-0bc7-4652-ab5f-9887d3dde558/zig-x86_64-macos-0.15.2/lib/std/std.zig:1:1: note: struct declared here
pub const ArrayHashMap = array_hash_map.ArrayHashMap;
^~~
referenced by:
    runBuild__anon_81076: /Users/runner/work/_temp/34017541-0bc7-4652-ab5f-9887d3dde558/zig-x86_64-macos-0.15.2/lib/std/Build.zig:2215:44
    dependencyInner__anon_75499: /Users/runner/work/_temp/34017541-0bc7-4652-ab5f-9887d3dde558/zig-x86_64-macos-0.15.2/lib/std/Build.zig:2195:29
    10 reference(s) hidden; use '-freference-trace=12' to see all references
Command failed with exit code 2.
Error: Process completed with exit code 2.