docs(spec): reconcile spec with shipped wire + front-door README + ADR-0034

[phall1]

Docs/markdown only — no Rust changes. Brings the spec docs back into agreement with the shipped code, refreshes the project front door, and adds one design ADR.

Spec / code coherence

• CONCEPTS.md: protocol version 0.3.0 -> 0.5.0 (verified against PROTOCOL_VERSION in crates/phux-protocol/src/lib.rs). Dropped the now-dead "current reality" paragraph — the collection-tier dissolution has shipped, so the leaked-verb caveat no longer holds.
• proto.md: spec version 0.4.0 -> 0.5.0. Documented the leaf str/bytes length prefix as a u32 big-endian count (matching Encoder::write_bytes / Decoder::read_bytes), kept distinct from the varint the field-tagged TLV envelope uses. Reconciled the ErrorCode catalog with the shipped #[non_exhaustive] enum: OUT_OF_TIER = 5 and UNSAFE_PASTE = 203 marked reserved / not-yet-emitted; 102/103 now read under the shipped SESSION_NOT_FOUND / WINDOW_NOT_FOUND names.
• appendix-encoding.md: added §2.1 spelling out the u32-BE leaf prefix vs the envelope varint.
• CHANGELOG.md: 0.5.0-draft.7 clarification entry. Core version stays 0.5.0, so spec-version-sync stays green. No wire bytes change — this corrects the spec to the code.

ADRs

• ADR-0015 and ADR-0021: amendment / superseded banners pointing at ADR-0030 (L2 collection tier dissolved), matching the ADR-0002/0006/0007 banner convention. The standing decisions (L1/L3 layering; client-side selector resolution) are preserved; only the "L2 will own the session" framing is marked superseded.
• ADR-0034 (Proposed, design only): kitty graphics / image passthrough through the cell renderer. Server forwards APC _G verbatim (phux-server::downsample::handle_apc) but the client re-renders cells only (attach::render), so images drop on repaint/reattach. Prefers the Kitty Unicode-placeholder approach with an APC re-emit fallback; notes the snapshot-replay and pixel-size-reporting prerequisites. No code in this ADR.
• ADR/README.md: added the missing 0031-0034 index rows.

README

Top-tier OSS front door in the existing deadpan / peer-thesis voice: centered banner with a logo placeholder (HTML comment), wordmark + tagline, only badges that resolve today (CI + license), HTML-comment TODOs for crates.io / version / Homebrew, a demo placeholder pointing at docs/assets/demo.gif, and honest install tiers — from source works today, prebuilt binary and Homebrew marked coming, no fake cargo install. The "What phux deliberately won't do" section is preserved.

Architecture (vocabulary only)

• module-structure.md: a note aliasing the legacy PANE_* frame names to the canonical TERMINAL_* frames per ADR-0016.

Verification

• just docs-check: green (83 files, 0 violations).
• PROTOCOL_VERSION, write_bytes/read_bytes (u32 BE), and the ErrorCode enum were read directly to confirm the spec now matches the shipped bytes.
• No crates/**, docs/consumers/tui.md, or docs/CONFIG.md touched.

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