refactor(core): use bumpalo for compiler allocations

[OpportunityLiu]

Summary

• allocate parser AST nodes from one bumpalo::Bump arena per compilation
• thread the arena through parser state and keep Monaco's persistent AST in a resettable arena
• remove the mimalloc feature and dependency from core, N-API, Python, and the lockfile
• keep formatter and emitter support for arena-backed AST nodes

Why

The compiler creates many small, short-lived AST allocations. The system allocator was noticeably slower than the previous mimalloc-enabled benchmark. Grouping those nodes in a compilation-scoped bump arena reduces allocation and teardown overhead without replacing the process-wide allocator.

Performance

Measured with crates/core/benches/compile.rs on examples/41_fib.mira, using the system allocator in both cases (--min-time 2 --sample-count 100):

Version	Median
Before	45.69 µs
After	39.69 µs

This is about a 13.1% improvement over the system-allocator baseline.

API impact

Compiler::parse now accepts the arena that owns returned AST nodes. AST-owned box fields use parser::AstBox, and AST types no longer implement Clone where arena-backed ownership makes an implicit allocator unavailable.

Validation

• cargo fmt --all -- --check
• cargo check --workspace
• cargo test --workspace
• cargo bench -p mira-core --bench main -- compile --min-time 2 --sample-count 100
• pnpm -r --filter "!@mirascript/website" run build
• pnpm -r run test

[codacy-production]

Not up to standards ⛔

🔴 Issues 1 high

Alerts:
⚠ 1 issue (≤ 0 issues of at least minor severity)

Results:
1 new issue

Category	Results
Security	1 high

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🟢 Metrics 0 complexity · 4 duplication

Metric	Results
Complexity	0
Duplication	4

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🟢 Coverage ∅ diff coverage · +0.10% coverage variation

Metric	Results
Coverage variation	✅ +0.10% coverage variation (-1.00%)
Diff coverage	✅ ∅ diff coverage

View coverage diff in Codacy

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (08a978d)	14034	13570	96.69%
Head commit (b264b8c)	24176 (+10142)	23401 (+9831)	96.79% (+0.10%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#6)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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TIP This summary will be updated as you push new changes.}