docs: adopt calendar versioning + dual-pace edge/stable channels (ADR-0004)

[ChrisonSimtian]

⚠️ Breaking change (versioning scheme). Fallout moves to calendar versioning YYYY.MINOR.PATCH and retires the v11 numbering. The headline work previously slated for "v11" now ships as 2026.0.0. The 11.0.x packages never shipped a clean stable release (all unlisted), so this strands no stable consumers; the legacy release/v10 line is unaffected (stays 10.x, security/critical fixes only). This PR is docs-only — it commits the project to the scheme; version.json flips in the follow-up PR.

What & why

Our contributor velocity is bimodal: an AI-assisted lane that ships fast, and a deliberate hand-coding lane. This establishes a release architecture that lets both paces run in parallel without divergent trunks — and is as much a social fix (nobody waits on anybody) as a technical one.

The keystone is ADR-0004. Highlights:

• Calendar versioning YYYY.MINOR.PATCH — mechanically valid SemVer (major = year), so NB.GV/NuGet/ordering are unchanged. Reconciles the "yearly breaking majors" goal with the strict-semver camp.
• main is the edge channel — fast/AI lane, intentionally unstable, daily date-stamped prereleases (2026.2.0-edge.<YYYYMMDD>.<h>) to GitHub Packages only (no nuget.org fan-out — consistent with why ADR-0001 made main non-publishing).
• release/YYYY stable trains — the slow crowd's domain; hardened deliberately; non-breaking minors/patches only after the cut. Breaking changes batch to the yearly major.
• [Experimental("FALLOUT0xx")] for opt-in unstable APIs that ride any channel — so "experimental" needn't mean a divergent fork. Promoting to stable = removing the attribute.
• Two-tier review — light/fast on edge, rigorous/unhurried on promotion to stable.
• Legacy release/v10 (+ hotfix/v10.x) coexists unchanged as a semver maintenance line; release/v11 retired (kept for archaeology).

ADR-0004 supersedes the versioning section of ADR-0001 and extends its channel model; the release-branch + multi-channel CD machinery and ADR-0002's nuget.org-opt-in policy are retained.

Changes

• docs/adr/0004-… — the decision (context, decision, consequences, alternatives incl. why not gitflow / separate experimental branch / date-as-core).
• docs/adr/0001 — amendment note; docs/adr/README.md — index updated.
• AGENTS.md — versioning/channels brief, critical-rules + PR-flow rewritten for target/YYYY + breaking-batched-to-year.
• docs/agents/release-and-versioning.md — branching/versioning/policy/PR-flow/pipeline sections.
• docs/branching-and-release.md — branch table, channel taxonomy, release/hotfix/cut runbooks (CalVer, v2026.x tags, v10 legacy paths).
• CHANGELOG.md — [Unreleased] heading → 2026.0 + lead entry.

Follow-ups (this is PR 1 of a sequence)

• PR 2 — version.json → CalVer + edge prereleases + multi-release-branch publicReleaseRefSpec.
• Cut release/2026; PR 3 — release.yml edge-from-main job + channel mapping; PR 4 — [Experimental] analyzer/convention.
• A GitHub Discussion linking this ADR (to bring the wider contributor group in) + Projects/labels setup.

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