Add two-phase borrow support

[itsfuad]

Goal

Support reserved-then-activated mutable call borrows without weakening reference-origin or exclusive &mut guarantees.

Semantics

• Every mutable reference call argument reserves during left-to-right evaluation: explicit &mut, implicit reborrow, mutable receiver, optional mutable reference, and indirect callable.
• Reads and shared borrows may overlap a reservation only when they end before activation.
• Mutation, move, free, overwrite, destruction, and nested mutable activation conflict with reserved storage.
• Activation occurs immediately before entering the callee and requires exclusive access.
• Both(&mut value, Read(&value)) is valid when Read returns an owned/copied value.
• Both(&mut value, &value) remains invalid because the shared reference enters the same call.
• Unequal concrete fields/fixed indexes may reserve independently; dynamic indexes and views remain conservative.

Architecture

• Keep reservation facts expression-local in existing ownership loanContext.
• Keep persistent NLL on existing ownership CFG.
• Keep origin overlap canonical in internal/semantics/place.
• Preserve ReturnOriginSources and reference-return composition.
• No syntax, ABI, HIR, MIR, backend, runtime loan state, or second CFG changes.

Validation

Passed on 2026-07-19 under 2 GiB process cap:

• Focused two-phase ownership tests, 20 repeated runs.
• Full go test -p 1 ./....
• Full go test -race -p 1 ./....
• Full go vet ./....
• Compiler bundle.
• Positive x_test/runtime_borrow_conflicts runtime fixture.
• Negative x_test/negative_borrow_conflict fixture with exactly seven intended T0037 errors and no unrelated diagnostics.
• gofmt -l, git diff --check, and executable-artifact scan clean.

Follow-up to #43.