Clean compiler phase boundaries

[itsfuad]

Summary

• preserve source-written HIR control flow through mandatory semantic checks
• represent uninitialized HIR bindings without fake invalid expressions
• finalize typed CFG edges and CFG diagnostics inside PhaseCFG
• move definite initialization from Resolver into CFG dataflow
• separate ownership cleanup output from immutable CFG
• add typed semantic export fingerprints and dependent invalidation
• clarify eager semantic const evaluation versus typed HIR folding

Validation

• GOCACHE=/tmp/peeper-go-cache CCACHE_DISABLE=1 go test ./... -count=1
• GOCACHE=/tmp/peeper-go-cache CCACHE_DISABLE=1 go vet ./...
• GOCACHE=/tmp/peeper-go-cache CCACHE_DISABLE=1 go test -race ./... -count=1
• GOCACHE=/tmp/peeper-go-cache CCACHE_DISABLE=1 go run ./scripts/bundle.go
• PEEPER_BIN=/home/itsfuad/Dev/Peeper/compiler/build/bin/peeper go test ./x_test -count=1

Tracking

Progresses #28 by establishing CFG and flow boundaries required for optional narrowing.

Future constraints

• Optional narrowing remains out of scope for this PR.
• No optimization may remove source-written control flow before mandatory semantic analyses finish.
• Ownership remains on typed HIR-derived CFG for now; Raw MIR ownership is a later architecture decision.
• Semantic API equality must continue to imply equal compiler-visible public interface.