Make UnitMap cheaply cloneable via Arc to reduce per-context allocation overhead #318

Context

UnitMap wraps BTreeMap<String, i32> and is constructed from scratch every time units::Context::new is called. WRLD3 has ~311 unit definitions, each producing a UnitMap entry.

In the current incremental compilation architecture, unit contexts are rebuilt per-variable (see related work to cache the context at project level). Even after that fix, UnitMap cloning during unit inference (units_infer.rs) remains allocation-heavy: the ConstraintSet::substitute and unify functions clone UnitMap values hundreds of times during inference (138K + 128K bytes in the base branch DHAT profile).

Proposal

Wrap the inner BTreeMap in an Arc:

#[derive(Clone, Debug, PartialEq, Eq)]
pub struct UnitMap(Arc<BTreeMap<String, i32>>);

This makes cloning O(1) (atomic refcount increment) instead of O(n) (deep copy of the BTreeMap). The UnitMap is immutable after construction, so shared ownership is safe.

Scope

• Change UnitMap internals to use Arc<BTreeMap<String, i32>>
• Update FromIterator impl to wrap in Arc
• Ensure Hash, PartialEq, Eq, PartialOrd, Ord impls dereference through the Arc
• Verify unit inference (units_infer.rs) benefits from cheap clones in ConstraintSet::substitute and unify

Acceptance Criteria

• UnitMap clone is O(1) via Arc
• cargo test -p simlin-engine passes
• cargo bench -p simlin-engine --bench compiler -- wrld3 shows reduced allocation count in project_build
• Unit inference tests pass with no behavioral changes
• DHAT shows reduced BTreeMap clone allocations in units_infer.rs

Notes

This is a targeted, low-risk optimization. UnitMap is never mutated after construction, making Arc a clean fit. The main risk is any code that currently pattern-matches or borrows the inner BTreeMap directly -- those sites need to dereference through the Arc.

Profiling data (WRLD3, base branch, single pipeline run):

• BTreeMap clone_subtree in UnitMap::clone: 138K + 128K bytes = ~266K total
• In unit inference unify/substitute loops: 833 allocations