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

[ChrisonSimtian]

⚠️Breaking change (versioning scheme). Fallout moves to calendar versioningYYYY.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.
• mainis 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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