Freeze conformance requirements per spec revision, run each at its wire (#447)

There is no way to answer "which scenarios must my implementation pass
to conform to spec revision X". The suite accumulates scenarios
continuously, so what an implementation is measured against keeps
growing after the revision shipped, and it can drop below 100% while
standing still. --spec-version does not answer it: it filters on which
revision a scenario targets, not on when the scenario became a
requirement.

Add requirements/<revision>.yaml naming the scenarios a revision
requires, and --requirements <revision> on the client, server, list and
sdk commands to run exactly that set. The revision also fixes the wire
version its scenarios speak: the dated revisions through 2025-11-25 use
the stateful initialize handshake and 2026-07-28 is stateless with
per-request _meta, so a scenario belonging to two revisions runs once
under each and one run does not cover the other. Passing no flag keeps
today's behaviour of running everything.

tier-check gains --sdk <name[@ref]> / --sdk-path <dir>: for each claimed
revision it resolves that revision's build, server invocation and client
command from the sdk-runner's KNOWN_SDKS specOverrides, manages the
server, runs both legs with cwd = the SDK checkout, and merges. Every
claimed revision must pass for Tier 1. This is what makes multi-revision
tiering correct for SDKs whose wire era is fixed per server process
(go-sdk only speaks >= 2026-07-28 when constructed stateless) or per
endpoint (csharp-sdk serves it at /stateless): one
--conformance-server-url cannot describe them, and their config already
does. The single-URL mode remains for SDKs not in the config.

Ship two sets. 2026-07-28 is a snapshot of 0.2.0-alpha.10, published the
day before that revision shipped. 2025-11-25 is a reconstruction from
the same anchor and its header says so: the release current on its ship
date predates spec-version awareness entirely. not_scored entries run
and are reported with a reason (extension per SEP-1730, or
added-after-release) but never affect a pass rate or the exit code, and
--expected-failures judges scored scenarios only. Reports label every
entry with its revision, and JSON passed/failed/total describe the
scored set so they always agree with pass_rate.

tier-check's exit code now gates on the machine-checkable half: nonzero
when a scored scenario fails or a leg could not be measured. A child run
that cannot happen at all (a set naming a renamed scenario, an
unreachable server) reports as not measured rather than as 0% against
the implementation. Conformance runs to completion before the GitHub
checks, so an API failure can no longer interrupt a leg and orphan a
managed server, and the legs spawn their child asynchronously: the
previous synchronous exec blocked the event loop, the parent stopped
draining the managed server's log pipes, and a chatty server (rust-sdk's
per-request tracing) froze on a full pipe and was scored as failing.

Driving every tier-1 SDK through the new path also fixed three configs:
csharp-sdk's server now launches from its build-output directory so
ASP.NET finds appsettings.json (its AllowedHosts filter, which
dns-rebinding-protection tests, silently never loaded from the repo
root); rust-sdk's STATELESS=1 override is removed because its CI runs
one default server for both revisions and the forced mode fails its
SEP-2575 scenarios; python-sdk's build mirrors its CI's two targeted
syncs because --all-packages is unbuildable on main. withManagedServer
now waits for its child to exit before returning, so consecutive runs on
one port cannot race a dying predecessor.

package.json

@@ -27,7 +27,8 @@
     "prepare": "npm run build"
   },
   "files": [
-    "dist"
+    "dist",
+    "requirements"
   ],
   "bin": {
     "conformance": "dist/index.js"