docs(assets): integrate M3 hub production flow

[colorpulse6]

Summary

• update the master roadmap to record G0 PR feat(galaxy): add the G0 continuous Atlas substrate #16 as merged and make M3 asset production part of the milestone
• add an execution-ready Cantina → Marketplace → Town Hall prompt pack and machine-readable 42-asset manifest
• add a copy-safe Fable implementation plan covering worktree setup, selected-source provenance, ImageMagick review evidence, validation, commits, and final handoff
• separate PRODUCTION-REVIEWED static art from later runtime registration and INTEGRATED gameplay proof

Why

M3 needs buildings, props, and NPC identities as part of the development flow—not a late polish pass. This contract lets an asset worker make substantial parallel progress without inventing hub gameplay, stretching non-square props through the current renderer, or claiming unused files are integrated.

Verification

• git diff --check origin/main...HEAD
• manifest/schema audit: 3 bundles, 42 unique asset slots (cantina: 14, marketplace: 15, town-hall: 13)
• authority-path audit: all 10 referenced live docs/runtime instruction paths exist
• confirmed the commit changes docs only; no game/ files are present in the diff
• two independent review passes: asset/runtime contract approved; Fable plan executability approved

Game tests/build were not rerun because this PR changes documentation and a JSON planning manifest only; it adds no production assets or runtime code.

Handoff boundary

This PR does not generate art or modify runtime consumers. The asset plan ends at static PRODUCTION-REVIEWED; a later approved Cantina code slice owns sprites.ts registration, hub data/consumer wiring, and the 480×854 production-export playtest.

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