Fold quoted-price-vs-executable-liquidity framing into the README roadmap

> Written to the standard set by #33 — read that issue first for the expected level of detail. ## What Add the "quoted price ≠ executable liquidity" framing and the market/routing/risk category taxonomy to the README's roadmap section, explicitly marked as not-yet-implemented. The roadmap currently lists v2–v5 by capability. The missing framing is: a quoted price from an order book or SEP-38 endpoint is not the same as executable liquidity at a given trade size. This distinction is foundational to everything the project measures — a corridor can show a good quoted price and still destroy value at size because the book is thin, the route runs through an intermediate, or the anchor's quote is stale. ## Why it matters A reader who sees "USDC → NGNC: 27% floor loss" without understanding *why* — when the quoted rate looks reasonable — misses the project's core insight. The taxonomy (market quality, routing efficiency, execution risk) gives readers a mental model for interpreting the numbers, and it belongs in the README alongside the layer model, not buried in a commit message or session log. ## Which layer **Layer 4 — documentation only.** No code, no new metrics. Language and framing only. ## Exactly where the code goes - **Modified file:** `README.md` — the Roadmap section (lines ~344–370) - Add a brief paragraph after the v2 entry explaining the quoted-price vs executable-liquidity distinction - Add the market/routing/risk category taxonomy as a bullet list under v2, explicitly marked "not yet implemented" ## Where the data comes from No data. This is editorial work based on the project's existing measurements and the README's own layer model. The framing originates from the session brainstorming that produced the four-layer model and the corridor measurement methodology. The data backing it is already in `docs/corridor-measurements.md` (the USDC/NGNC 27% floor loss at 0.1 USDC, 97.68% at 5000). ## What "unmeasurable" means here Nothing is measured. This is a docs-only change. ## Acceptance criteria - [ ] The "quoted price ≠ executable liquidity" distinction is stated in plain language in the README - [ ] The market/routing/risk taxonomy is listed under v2, explicitly marked not-yet - [ ] The framing references the existing layer model rather than introducing new terminology - [ ] No code changes — README only - [ ] The prose matches the README's existing voice (direct, no hedging, no marketing language) ## Out of scope Do not change: - `docs/corridor-measurements.md` — the measurement records stay as they are - Any Go code - The layer model itself (it is already correct) ## How to verify locally ```bash # Read the README and confirm the framing is present and accurate cat README.md | grep -A5 "quoted price" cat README.md | grep -A5 "executable liquidity" ``` No build or test needed — this is a markdown-only change.