Add shared tagged variant core

[itsfuad]

Summary

• lower optionals and future enums through one tagged-variant descriptor and IR operation family
• replace optional-specific HIR, MIR, place, flow, and LLVM machinery with case-aware variant operations
• preserve ?T/none, optional ownership rules, and exact tagged optional ABI
• add exhaustive variant switching, typed payload slots, active-case drops, and declaration-owned nominal identity
• preserve issue Add optional narrowing and payload access #28 narrowing behavior, including alias invalidation and eager boolean evaluation

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

• no public enum construction or matching syntax in this PR
• general named-type applications follow in a separate stacked PR
• compact union payload storage remains deferred
• optional niche layout remains Add optional niche layouts #30

Stacked on #87. Does not merge or close #28.