[ITG] Add modules for ITG graph

[yushan8]

Why?

Implement data structure modules for ITG. The OptimizedGraph structure provides fast look and calculation of reverse dependencies of a target to incrementally calculate the full target graph.

What?

New package: core/itg/graph

Data structures (graph.go)

• IntSet / StringSet — map-based set types with Insert, Delete, Contains, UnsortedList, Copy
• OptimizedTarget — compact representation of a Bazel target with integer IDs for deps, rule types, tags, and
  attributes instead of strings (reduces memory)
• OptimizedGraph — the full dependency graph with bidirectional ID↔name mappings for targets, rule types,
  tags, and attributes
• OptimizeGraph() — converts a map[string]*targethasher.Target into an OptimizedGraph, wiring reverse deps
  automatically
• Copy() on both types — deep copies for safe mutation
• GetReverseDepsAsTargets() — DFS traversal returning a target and all its transitive reverse dependents
• OptimizedTargetToTarget() — converts a stored target back to targethasher.Target

Graph mutation (invalidate.go)

• removeTarget — removes a target, cleans up dep/reverse-dep cross-references, promotes deps to root if
  unblocked
• invalidateHashRecursively — walks reverse deps and sets Hash = nil (marks them as needing recomputation)
• checkDeletedTargets — removes targets whose packages were deleted or are no longer in a query result
• upsertTarget — inserts or updates a target, swapping dep references and invalidating affected hashes

Hash recomputation (update.go)

• UpdateGraph() — top-level entry point: translates a raw Bazel query result into updated targets, upserts
  external rules, computes available hashes, invalidates stale entries, then recomputes hashes in topological
  order
• computeAvailableHashes() — computes hashes for source files (via SourceHasher), package groups (sha1 of
  name), and rule targets (sha1 of rule attributes, without deps)
• computeHashes() — recursively computes final hashes: source files and package groups use their pre-computed
  hashes, generated files inherit from their parent rule, and rule targets combine their HashWithoutDeps with
  sorted dep hashes

Test Plan

unit test

Issue

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