[docs] Rewrite Transparent install flow as shipped architecture, drop phase tags

[bdraco]

What does this implement/fix?

Rewrites docs/ARCHITECTURE.md's Transparent install flow section as shipped architecture rather than a roadmap, and strips planning-doc phase tags (5b, 5c-2a, 5c-2b, 5c-3, 5d, 6a, 4a-r1, 4a-o, 7a-N, 7b, 8a, 8b, phase 3a) from the wider Remote-build section. Also drops the now-removed cert + key files from the persisted-state table.

Every slice in the original issue #106 plan has shipped — BuildScheduler (#553), esphome_version handshake (#557), FirmwareJob.source (#556), synthetic-job framing reversal (#558), source-routed runner (#560), install-handler integration (#568), e2e coverage (#573), Settings backend toggles (#574), compile → upload seam reset (#580). Open follow-ups ("force fallback to local", "allow major-version mismatch") land through the normal bug-fix process from here. The doc should reflect that — readers don't need to decode planning-slice shorthand to understand how the code actually runs.

Notable changes

• ### Transparent install flow — six "Slice 7a-N / 7b" prescriptive paragraphs collapsed to present-tense architecture prose. PR refs kept for traceability (Add BuildScheduler primitive (7a-1) #553, Add FirmwareJob.source / source_label fields (7a-2a) #556, Drop synthetic-job / event-stream-bridge framing for 7a-2+ #558, Source-route firmware runner on JobSource.REMOTE for compile (7a-2b) #560, Route firmware/install through pick_build_path + REMOTE upload chain (7a-3) #568, Add e2e tests for 7a-3 install scheduler routing + dual-flow round-trip #573, Add 7b backend: per-pairing enabled + master remote_builds_enabled toggles #574, Reset firmware-job progress at compile→upload seam on REMOTE installs #580). "Not yet implemented" subsection replaced with a one-line note on the deferred items.
• Receiver / offloader dispatch paragraphs — (5b — …), (5c-2a — …), (5c-2b — …), (5c-3 — …), (5d — …), (6a — …) annotations stripped from parentheticals; functional descriptions unchanged.
• Endpoint rebind subsection — (4a-o part 7, #539) → (#539), (8b, #548) → (#548), "8a re-auth wizard" → "re-auth wizard".
• Listener internals — Identity bullets — replaced cert + dashboard_id bullet with dashboard_id + X25519 peer-link keypair only. The cert is gone in code.
• Persisted state table — dropped .device-builder-cert.pem and .device-builder-key.pem rows. The files aren't written anymore; tests at tests/test_dashboard_identity.py:242-243 and tests/test_remote_build_controller.py:1678-1679 explicitly assert their absence. Backup-tools paragraph now references .device-builder-peer-link-key.bin + rotate_identity.
• Other — Remote build intro paragraph's "phases 3a–3c shipped… wound down across phase 4a-r1/4a-r2…" historical recap replaced with a one-liner.

Net: +28 / −30 lines, 0 phase tags remaining in the doc.

Related issue or feature (if applicable):

• documentation cleanup; no behavioural change.

Types of changes

• Bugfix (non-breaking change which fixes an issue) — bugfix
• New feature (non-breaking change which adds functionality) — new-feature
• Enhancement to an existing feature — enhancement
• Breaking change (fix or feature that would cause existing functionality to not work as expected) — breaking-change
• Refactor (no behaviour change) — refactor
• Documentation only — docs
• Maintenance / chore — maintenance
• CI / workflow change — ci
• Dependencies bump — dependencies

Frontend coordination

• No frontend change needed
• Companion frontend PR: esphome/device-builder-dashboard-frontend#

Checklist

• The code change is tested and works locally.
• Pre-commit hooks pass (ruff, codespell, yaml/json/python checks).
• Tests have been added or updated under tests/ where applicable.
• components.json has not been hand-edited (regenerate via script/sync_components.py if a sync is needed).
• Architecture-level changes are reflected in docs/ARCHITECTURE.md and/or docs/API.md.

[codspeed-hq]

Merging this PR will not alter performance

✅ 12 untouched benchmarks

---

_{Comparing docs-strip-phase-tags-and-cert-refs (cbef86d) with main (e459c2b)}

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.21%. Comparing base (e459c2b) to head (cbef86d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #606   +/-   ##
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  Coverage   99.21%   99.21%           
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  Files          83       83           
  Lines       10653    10653           
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  Hits        10569    10569           
  Misses         84       84           

Flag	Coverage Δ
py3.12	99.18% <ø> (ø)
py3.14	99.21% <ø> (ø)

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[Copilot]

Pull request overview

Updates docs/ARCHITECTURE.md to describe the shipped remote-build / transparent-install architecture (rather than a phased roadmap), removing phase-tag annotations and reflecting the current persisted identity/state layout.

Changes:

• Rewrites the “Transparent install flow” section into present-tense architecture prose and removes “in-flight”/phase-slice framing.
• Removes phase-tag parentheticals across the remote-build section and simplifies some historical context.
• Updates identity + persisted-state documentation to drop the legacy cert/key files and focus on the X25519 peer-link identity.