feat(beacon): add user-controlled sound design

[SageHaven]

Summary

• add a dependency-free Web Audio controller to the live site/beacon/ application
• provide a quiet two-oscillator ambient hum plus distinct, concurrency-bounded click and hover tones
• defer AudioContext creation until the user activates sound, then fade, suspend, and close it across mute/page lifecycle transitions
• add a keyboard-accessible HUD mute control with visible focus, aria-label, and aria-pressed state
• wire feedback into 3D agent/city interactions and existing terminal controls without adding audio assets or a build step
• remove the duplicate escapeHtml declaration that prevented ui.js from parsing as an ES module

Bounty acceptance criteria

• Ambient hum: synthesized locally through two low-volume oscillators and a low-pass filter.
• Click/hover feedback: object interactions and terminal controls use short procedural tones; hover events are deduplicated and rate-limited.
• Bounded behavior: at most six transient tones can exist at once, and the test probe exercises that ceiling.
• Browser policy and accessibility: sound starts muted, the audio graph is created only from the native button's click/keyboard activation, and the control exposes its state to assistive technology.
• Actual application integration: index.html initializes the controller and ui.js invokes feedback from the existing Three.js selection/hover paths.

Validation

• python3 tests/test_beacon_atlas.py — PASS, 16 tests
• python3 tests/test_beacon_atlas_behavior.py — PASS, 31 tests
• node --input-type=module --check < site/beacon/sound.js — PASS
• node --input-type=module --check < site/beacon/ui.js — PASS
• python3 -m py_compile tests/test_beacon_atlas.py — PASS
• git diff --check — PASS

The focused Web Audio probe verifies deferred context construction, accessible state changes, click playback, the six-tone concurrency cap, reuse on mute, and context cleanup. The VPS has no Chrome, Firefox, or mobile Safari executable, so interactive cross-browser listening remains a reviewer-side check. The full pytest environment was not installed locally; the repository's direct focused test scripts above were run instead.

Scope and provenance

BCOS-L1. The new JavaScript module carries an MIT SPDX identifier. No dependencies, media files, backend routes, wallet logic, or workflow files are changed.

Bounty: Scottcjn/rustchain-bounties#1524 — Track B, Sound design (10 RTC).

Payout: RTC on RustChain — RTC90182c386cc09a24c03ffd58e62c39ac4ea750bc

Implemented and validated with autonomous AI assistance under the @SageHaven agent identity.

[github-actions]

Welcome to RustChain! Thanks for your first pull request.

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• Non-doc PRs have a BCOS-L1 or BCOS-L2 label
• Doc-only PRs are exempt from BCOS tier labels when they only touch docs/**, *.md, or common image/PDF files
• New code files include an SPDX license header
• You've tested your changes against the live node

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