refactor(llm-access-kiro): split cache_sim.rs into submodules

[acking-you]

What

Split the 2190-line llm-access-kiro/src/cache_sim.rs into a cache_sim/mod.rs
facade plus 4 concern-focused submodules. Pure structural move — no logic
changes. This is the next step in the incremental llm-access* refactor
(follows #4 which split request.rs).

Module layout

 ConversationState
    -> [projection]   canonical token pages + conversation anchor segments
    -> [prefix_tree]  radix trie of shared stable-prefix pages
    -> [anchor_index] LRU conversation-id recovery keyed by anchor hash
    -> [simulator]    KiroCacheSimulator ties the three together

File	Holds
projection.rs	PromptProjection, RuntimePromptProjection, CanonicalTokenPage, PREFIX_CACHE_PAGE_SIZE, all canonicalize_*/tokenize/hash helpers + segment structs
prefix_tree.rs	PrefixTree (+ node/edge internals & free-fn helpers), PrefixCacheMatch, PrefixTreeRuntimeStats
anchor_index.rs	ConversationAnchorIndex (+ entry/estimate helpers), ConversationAnchorRuntimeStats
simulator.rs	KiroCacheSimulator, KiroCacheSimulationMode, KiroCacheSimulationConfig, KiroCacheRuntimeStats

Module style (per the agreed standard)

• mod.rs convention (matches the rest of the workspace).
• No use super::* in submodules — each imports deps explicitly from their
  origin. No pub use X::* globs: mod.rs re-exports only the 10 items
  used outside the module, by name, so the cache_sim::X public surface
  (consumed by llm-access provider.rs/admin.rs and kiro config.rs) is
  unchanged.
• Minimal visibility: items crossing a module boundary (PrefixTree + 4
  methods, ConversationAnchorIndex + 5 methods, PREFIX_CACHE_PAGE_SIZE) are
  pub with a /// doc; file-internal helpers/structs stay private. No
  pub(crate).
• Tests inlined: 5 projection unit tests → projection.rs, 7 prefix-tree unit
  tests → prefix_tree.rs (so the private items they exercise need no extra
  exposure); the 4 simulator integration tests stay in mod.rs against the
  public API.

Verification

• Item inventory diff vs original: 0 dropped, 0 added structural items;
  #[test] count 16 = 16.
• cargo clippy -p llm-access-kiro -- -D warnings: clean.
• cargo test -p llm-access-kiro: 160 passed.
• Reverse-dep cargo build -p llm-access: compiles against the unchanged
  public surface.
• rustfmt on the 5 new files only; deps/lance & deps/lancedb stayed clean.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request refactors the Kiro prefix-cache simulation module by splitting a single monolithic file into focused submodules: anchor_index.rs, prefix_tree.rs, projection.rs, simulator.rs, and mod.rs. The feedback highlights an opportunity to optimize build_resume_anchor_hash in projection.rs by updating the Sha256 hasher directly, thereby avoiding unnecessary string clones and memory allocations.