feat: add solver views for slash cooldown, active intents, per-token stats, routes

[had3sgames]

Summary

Implements the 4 open issues assigned to @had3sgames as one combined PR:

• [High] Add a get_slash_cooldown_remaining convenience view #256 — get_slash_cooldown_remaining(solver) -> u64: seconds left in a solver's post-slash SLASH_COOLDOWN, sharing the exact comparison accept_intent uses via a new internal slash_cooldown_remaining helper so the two can never disagree.
• [High] Add a get_solver_intents view enumerating a solver's accepted intents #245 — get_solver_intents(solver) -> Vec<BytesN<32>>: enumerates a solver's currently Accepted intent ids via a new SolverIntents list, kept in sync in accept_intent (append), fill_intent (removed on both the full-fill and partial-fill branches, since either way the solver relinquishes ownership), and slash_solver (removed).
• [High] Add per-dst-token cumulative volume and fee tracking #246 — get_token_stats(token) -> (i128, i128): per-dst_token cumulative volume/fees, incremented in fill_intent alongside the existing global TotalVolume, on every partial fill.
• [High] Add a solver per-route preference/eligibility filter #255 — set_solver_routes / get_solver_routes: optional, purely advisory per-solver src_chain/dst_token route declaration, bounded by a new MAX_ROUTE_ENTRIES (20) cap and a new Error::TooManyRouteEntries variant. accept_intent does not enforce it.

docs/solver-integration-guide.md (recovery flow, crash recovery, route declaration) and the README.md error table are updated to match.

Important note on validation

This environment has no Rust/cargo toolchain, so cargo check/cargo test could not be run. While reviewing, I also found that the intent_settlement crate on main currently fails to build independently of this change: several constants and Error variants referenced throughout lib.rs and test.rs — e.g. CANCEL_COOLDOWN, MAX_PROTOCOL_FEE_BPS, MIN_FILL_WINDOW_SECS, MIN_INTENT_EXPIRY_SECS, MIN_BOND_FLOOR, MAX_BATCH_SIZE, MAX_EXTENSION_DURATION, DEFAULT_MIN_BOND/DEFAULT_FILL_WINDOW/DEFAULT_INTENT_EXPIRY/DEFAULT_PROTOCOL_FEE_BPS, Error::InvalidConfig, Error::AmountTooLarge — are used but never defined. They appear to have been dropped from main in a prior merge (they exist in commit 2e36dee but not in the current tree). This predates and is unrelated to these 4 issues, so I left it untouched per scope, with one exception: SLASH_COOLDOWN was restored (as a const) since get_slash_cooldown_remaining (#256) directly depends on it and the issue text assumes it already exists.

Please run CI on this PR to confirm, and consider a separate fix for the missing-constants regression on main.

Test plan

• CI build/test (no local toolchain available to pre-validate)
• Manual review of accept_intent/fill_intent/slash_solver bookkeeping changes for SolverIntents and per-token stats

Closes: #245
Closes: #246
Closes: #255
Closes: #256

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