Add generic named type instances

[itsfuad]

Summary

• parse explicit named-type applications, nested type arguments, imported applications, and multi-segment qualified paths
• add declaration-owned type parameters and one canonical substitution path for structs, enums, interfaces, and transparent aliases
• cache concrete instances by declaration and semantic argument identity, including recursive pointer/reference shells and owner-module reset
• preserve nominal enum identity while retaining existing struct and transparent-alias compatibility
• carry concrete applications through typechecking, HIR, MIR, LLVM, LSP hover, and bundled source fixtures
• reject type arguments on value paths because generic functions and values remain deferred

This PR is stacked on #88. It does not add public named-enum declarations, construction, is tests, or match syntax.

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

Deferred work

• Add contextual enum variant construction and inference #94 contextual variant construction and generic argument inference
• Compact tagged variants into shared union payload storage #92 compact shared union payload storage
• Add optional niche layouts #30 optional niche layout
• Add Polonius-like location-sensitive loan analysis #93 location-sensitive loan analysis
• Add value-producing match expressions #89 value-producing match expressions
• Add advanced enum match patterns #95 advanced match patterns
• Add enum protocols and metadata operations #91 enum protocols and metadata operations
• Define explicit enum representation and foreign ABI #96 explicit representation and foreign ABI
• Add advanced generics for functions and methods #90 generic functions, methods, constraints, inference, and monomorphization

Rules check

No pass-through compatibility wrappers, stale aliases, ignored parameters, duplicate type walkers, or parallel lowering paths were added. New path, substitution, traversal, and cache helpers centralize shared semantic invariants.