Add named enums and exhaustive match

[itsfuad]

Summary

• add payload-aware named enum declarations with declaration-owned variant symbols and generic substitution
• construct fully named local and imported cases through canonical semantic evidence and shared tagged-variant lowering
• add nonconsuming is tests, case-set CFG narrowing, exact-case fields, and alias-aware invalidation
• add exhaustive statement match with arm scopes, case edges, definite initialization, ownership transfers, and cleanup evidence
• preserve carrier ownership for copyable and shared-reference fields while rejecting move-only partial extraction
• add structured variant constants, typed MIR statics, LLVM aggregate emission, enum FFI rejection, LSP support, fixtures, and documentation

Validation

• CCACHE_DISABLE=1 GOCACHE=/tmp/peeper-go-cache go test -count=1 ./...
• CCACHE_DISABLE=1 GOCACHE=/tmp/peeper-go-cache go vet ./...
• CCACHE_DISABLE=1 GOCACHE=/tmp/peeper-go-cache go test -race -count=1 ./...
• CCACHE_DISABLE=1 GOCACHE=/tmp/peeper-go-cache bash scripts/build.sh
• PEEPER_BIN="$PWD/build/bin/peeper" CCACHE_DISABLE=1 GOCACHE=/tmp/peeper-go-cache go test -count=1 ./x_test
• git diff --check

Boundaries

• variants require fully named Enum::Variant syntax and never become types or callable constructors
• struct literals remain .Point{...} and .{...}
• match remains statement-only, exhaustive, and fully named; no wildcard, guard, alternative, nested, shorthand, or expression forms
• move-only field extraction remains legal only from direct named local or parameter carriers; partial carriers are rejected
• LLVM keeps one typed payload slot per data case; compact union storage remains Compact tagged variants into shared union payload storage #92
• named enums remain rejected in extern signatures until Define explicit enum representation and foreign ABI #96

Stacked on #97. Parent PRs #87, #88, and #97 remain open and unmerged.

Deferred work: #30, #89, #91, #92, #94, #95, and #96.