feat(snowflake): Milo substrate flavor + prototype interaction layer

[vhargrave]

Add a Milo substrate flavor to the snowflake skill

Problem

On a Milo target repo (e.g. adobecom/da-playground), the snowflake skill produces a broken page: the global nav renders as a ~4806px, fully-expanded, unstyled blob.

Root cause: install-substrate.mjs replaces head.html + scripts/scripts.js (and the header/footer blocks) with the EDS overlay substrate. On a Milo repo that rips out the Milo runtime — the thing that loads the live global-navigation + footer from gnav-source/footer-source metadata. With Milo gone, the capture step freezes the rendered gnav DOM into a static fragment, and with no global-navigation.js to collapse it, every mega-menu paints expanded.

Fix — a second substrate flavor, auto-detected

The EDS flavor is unchanged. A new Milo flavor is selected automatically when the target boots milolibs (head.html contains milolibs, or scripts/scripts.js calls setLibs()); override with --flavor=milo|eds / --milo / --eds.

The Milo flavor is minimal and additive: it installs only blocks/snowflake/{js,css} and leaves Milo's head.html, scripts/scripts.js, and styles/styles.css byte-identical. Milo keeps ownership of the chrome (live gnav/footer from metadata); the bespoke body is drawn by the snowflake overlay block, which Milo loads from the project's codeRoot. The block reuses the EDS overlay logic verbatim (readBlockSlots / writeSlot 5-case / <link> lifting / slot application) — just relocated from scripts.js into a block so it coexists with Milo instead of replacing it.

What's in the PR

• assets/substrate-milo/ — MANIFEST.json (minimal replace: blocks/snowflake only), blocks/snowflake/{js,css}, VERSION.
• scripts/install-substrate.mjs — flavor auto-detection + --flavor/--milo/--eds; resolves the bundle dir by flavor; stamps substrateFlavor in .snowflake/config.json. Existing backup/idempotency/drift logic untouched (it's manifest-driven).
• phases/0,1,3,4 — Milo deltas: document flavor detection; capture chrome metadata (not chrome DOM); emit a Milo DA page (empty <header>/<footer>, one snowflake block, a metadata block re-emitting foundation/gnav-source/footer-source/unav/universal-nav + template + title); Wire copies only templates/<t>.html + styles/<t>.css, no fragments, no head/scripts changes.

Verification

• Real install on a Milo fixture: Milo files byte-identical before/after; only blocks/snowflake/* + ignore patches added; substrateFlavor: "milo" stamped. EDS flavor still selectable (no regression).
• Headless overlay-block test (10/10): overlays <main> from the template, applies slot overrides, keeps template defaults, injects static sections, consumes the authoring block, lifts the template's <link>s, injects per-template CSS, and leaves <header>/<footer> untouched.

Prototype interaction layer

The overlay injects pre-decorated, frozen DOM, so interactive widgets (carousel/slider, auto-rotating marquee, tabs, accordion) render but don't move. The snowflake block ships a tiny dependency-free activator that revives them via an explicit proto-* contract (.proto-carousel / .proto-marquee / .proto-tabs / .proto-accordion), with mechanics-only CSS scoped under [data-overlay]. Phase 3 instructs the generate step to rewrite captured widgets to the contract only when the capture retained every state — true for URL/HTML captures, which serialize the full rendered DOM. This is block-agnostic: snowflake just carries the contract markup through; it never decorates or understands a Milo block.

Capture-mode limit (important): a Figma-sourced prototype converges to a single static reference image, so off-screen slides never exist. Single-frame widgets are left static and never fabricated — the contract is for full-DOM captures, not Figma. This is prototype-grade motion (slide/advance/toggle), not a 1:1 reimplementation of each C2 block's physics; a true-Milo-blocks conversion remains a separate effort.

Notes / out of scope

• unav: on needs imsClientId in the host's Milo config to fully hydrate profile/cart; the gnav itself renders regardless.

Builds on the snowflake skill merged in #154. Based against main; touches only the new substrate-milo/ bundle plus additive Milo deltas to install-substrate.mjs and phases 0/1/3/4 — all of which are unchanged in main since this work began, so it merges cleanly. (Note: open PR #161 reworks the substrate mechanism; if it lands first this will be rebased.)