docs: move the JIT system guide to the solver crate

[kayibal]

The JIT document lived at contracts/README.md as if the system were contracts only. Most of it
is now Rust, so it moves to where the crate lives and picks up references to the code.

What moved

contracts/README.md -> crates/fynd-jit-solver/README.md (559 lines), next to the code it
describes and alongside crates/intent-backrunner/README.md, the sibling mode's guide. The root
README's JIT link now points there.

What the new contracts README covers

A contracts reviewer's document, 155 lines, scoped to FyndJITRouter, RouteDictionary and
RouteDecompressor: the external surface (user entrypoints, storeRoute / storeBatch, the
views, the admin functions), the invariants that matter for review, the bit-stream layout, the
deploy script and its role grants, and the forge suites including the Base fork suite. No
economics and no architecture narrative; those live in one place now.

What was added about the Rust side

• A module table for crates/fynd-jit-solver/src/{book,decode,engine,quoter,route_delta,gate,l1fee,compress,dictionary,decisions,state}.rs
  and one for crates/route-compressor/src/{codec,bitstream,segment,order,dictionary,contracts}.rs.
• The five stages of a solve pass, with the function behind each.
• The rules a reader cannot infer: the baseline is the order's own expectedAmountOut decoded
  from calldata, with no eth_call and no simulation of the original route; the surplus rule is
  the only emission rule; the same-route rule refuses a byte-identical-after-normalisation route
  as same_route, with same_pools_new_split flagged but still gated; the builder tip is
  charged in the gate because Fynd pays it; the L1 data fee is auto-detected via the OP-stack
  GasPriceOracle and is structurally zero without it; native tokens are quoted as the zero
  address while the router's convention is the 0xEeee… marker.
• A "Running the solver" section with the flags main.rs defines, including the three whose
  environment variables are not named after the flag, and what a builder must supply.
• The quote-trust limitation from quoter.rs, with --min-tvl as the operator's dial, kept
  alongside the existing counterfactual and compromised-key limits.

The economics section is preserved as rewritten in 1043b74: single-route framing, the four
break-even numbers, the FastLZ minimum-charge finding. The 2026-08-14 11:56 UTC prices were not
re-fetched, so the timestamp is unchanged.

Stale content fixed at this commit

• crates/fynd-jit-solver and its anvil integration test are merged, so all "under review in
  PR feat: fynd-jit-solver solve loop, profitability gate, and emission #19" and "not on this branch" phrasing is gone.
• The old text said the gate documents the L1 data fee as "routinely dominating". 2399ee2
  corrected that framing: L2 execution was about 274x the L1 term in the gate's own snapshot,
  and cheap blobspace is why.
• The Rust suite is 256 passed / 3 ignored, not "102 passed, 2 ignored".
• The old text claimed fixture regeneration leaves git status clean. It does not: the
  generator writes JSON keys in declaration order and the committed files are sorted, so all
  four fixtures come back modified with identical values.
• Warm-cache forge test is 20 to 30 seconds, not 8, and the fork suite's RPC round trips are
  nearly all of it.

Verification

forge test from contracts/: 107 passed. cargo test --workspace: 256 passed, 3 ignored;
fixture churn dropped with git checkout -- contracts/. Every relative link resolves from its
file's directory and all 64 cited code symbols exist in the tree.

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