Add minimal browser UI and update prover documentation (#100)

## Summary

This PR introduces a self-contained browser UI for proof submission and
updates documentation to reflect the actual prover surface exposed by
the REST API. The changes clarify that while ECHIDNA implements 100+
prover backends, the default API surface is deliberately limited to a
12-prover core set.

## Key Changes

- **New file: `src/ui/public/prove.html`** — A minimal, self-contained
REST client UI for proof submission
- No build step required; opens directly in any modern browser via
`file://`
- Connects to a running ECHIDNA server (e.g., `cargo run -- server
--cors`)
- Supports prover selection, timeout configuration, neural assist toggle
- Displays proof results with elapsed time, goal count, and outcome
status
  - Includes example SMT-LIB snippets (Z3 SAT/UNSAT) for quick testing
  - Dark theme with GitHub-inspired color palette

- **New file: `docs/SUPPORTED_PROVERS.md`** — Authoritative
documentation of prover support
  - Clarifies the 12-prover core set exposed via `GET /api/provers`
- Documents required external binaries and installation hints for each
core prover
- Explains the distinction between core (12), advertised (67), and total
(141) prover variants
  - Provides quickstart instructions for proving the first goal with Z3
  - Describes how to expose additional backends through the REST API

- **Updated: `README.adoc`**
  - Revised prover badge from "128" to "12 core / 67 advertised"
  - Updated test count from "917+" to "1059"
- Replaced "Leadership Position" section with "Prover Surface" and
"Distinguishing Features"
  - Simplified prover tier table to focus on the 12-core set
  - Added "Prove your first goal" quickstart section with curl examples
- Updated architecture documentation to reflect 102 backend
implementation files
  - Clarified test suite counts and VeriSim integration

- **Minor: `src/rust/corpus/octad.rs`** — Refactored hex formatting
- Extracted `to_hex()` helper function to avoid repeated
`format!("{:x}", ...)` patterns
- Applied consistently across `octad_key()`, `cross_prover_key()`,
`content_hash_of()`, and `hash_provenance()`
  - Improves code maintainability and readability

## Implementation Details

The browser UI (`prove.html`) is fully self-contained with:
- Inline CSS (dark theme with semantic color variables)
- Vanilla JavaScript (no dependencies)
- Health check polling on API base change
- Prover roster auto-loading from `/api/provers`
- Real-time result display with outcome classification (ok/bad/warn)
- Example snippets that auto-populate content and prover selection

The documentation updates align the public-facing narrative with the
actual REST API surface, reducing confusion for new users while
preserving the full 100+ prover capability for Rust API consumers.

https://claude.ai/code/session_01RLu1DN3tSHfaVc8aqTd7VZ

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: hyperpolymath

.machine_readable/6a2/ANCHOR.a2ml

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: MPL-2.0
 # ⚓ ANCHOR: rsr-template-repo
 # Canonical authority and project recalibration trigger for rsr-template-repo.
 # Template from: https://github.com/hyperpolymath/standards

.machine_readable/contractiles/bust/bust.k9.ncl

@@ -51,6 +51,10 @@ let base    = import "../_base.ncl" in
       allow_filesystem_write = true,      # drills may write transient state (tmp dirs, test DBs)
       allow_subprocess = true,
       injection_scope = 'system_under_test_only,
+      # Hunt-tier files must declare whether a cryptographic signature
+      # is required. Bust trident runs in CI on every PR; signatures
+      # are enforced at the trident-runner layer, not in-band.
+      signature_required = true,
     },
   },