feat(web): Mixdown — interactive lab of the muzix closed loop

[abhicris]

Summary

Adds Mixdown — an interactive canvas lab that walks viewers through the entire muzix closed loop in six animated scenes. Single static HTML, no build step, fork-and-extend friendly. Lives at `web/public/mixdown.html` → served at `/mixdown.html` in production (`muzix.kcolbchain.com/mixdown.html`).

Mirrors the pattern from `kcolbchain/switchboard/web/agents-demo.html` but in muzix's vernacular — gold accent, music-flavored scene names, the contracts shipped this week as the cast.

The six scenes

#	Scene	What it shows	Contract calls
1	Drop the master	Sapta registers an album; cap-table (60/20/20) commits; root variant mints as 10,000 ERC-1155 units fanned by bps	`Labelton.registerMaster`
2	Term sheet on the wire	Sapta drafts a 3-yr exclusive distribution offering and publishes it	`MuzixRightsOffering.createOffering` + `publishOffering`
3	Counters in the room	Universal counters higher upfront; Sony takes base terms; Indie counters higher royalty — each posting 5k MUSD bond	`submitCounter` / `acceptBaseTerms`
4	Sapta picks	She accepts Sony; losing bidders self-withdraw and recover bonds	`acceptCounter` + `withdrawCounter`
5	The numbers roll in	Spotify / Apple / YouTube pushers submit verified revenue with confidence scores	`MuzixStreamingOracle.submitRevenue`
6	Mixdown pays out	MUSD reads `capTableOf` and fans the period revenue 60/20/20 atomically	`MUSD` + `Labelton.capTableOf`

Why "Mixdown"

It's the recording-studio step where every track lands in one final master. Same shape as what these contracts do together — negotiation, registration, revenue, and settlement all land in one place. Picks up the music vernacular without overlapping switchboard's "agents-demo lab".

Chrome

• Transport bar: play / pause (space), step (n), restart (r), arrow-key scene nav, prev/next buttons.
• Scene pill bar: click any scene to jump.
• Tooltip on hover: every agent shows its role, MUSD balance, or ERC-1155 share count.
• `+ add your scene` placeholder links to the file on GitHub.
• `web/public/SCENES.md` documents the scene contract so new scenes are an array append.

What's intentionally out of scope (follow-ups)

• AI training license scene (IPTO calling `MuzixRightsOffering` for sync-style data deal).
• Remix-variant scene (`Labelton.mintVariant` from a cap-table member).
• Catalogue-wide bundle scene (5-yr full rights instead of single-album distribution).
• Linking Mixdown from the Next.js app's primary nav — separate small PR.

Test plan

• HTML loads without console errors (syntax-checked the embedded JS via `node --check`)
• All 6 scenes registered; `SCENES.length === 6`
• Visual review — open at `/mixdown.html` after preview deploy and walk through all scenes
• Mobile layout sanity (caption + transport overlap check at 375 px width)