feat(backend): assemble the Forge runtime

[sandersonstabo]

Summary

• assemble the real Forge process around ForgeApp, loopback QUIC, credential custody, readiness publication, cancellation, and ordered shutdown
• replace the no-op backend entry point with explicit configuration and typed process exit outcomes
• register a real-crate runtime suite covering configuration, credential custody, readiness identity, service/drain failures, cleanup precedence, and restart behavior
• preserve the preceding Statig dependency graph and repin the combined Crate-Universe checksum

Verification

• //tests/backend:forge_runtime_test: 13/13 passed
• direct Clippy: //modules/backend:backend and //tests/backend:forge_runtime_test passed with fresh markers
• //:format_test: passed across the exhaustive aggregate
• Cargo metadata: passed
• immutable gate receipt at exact head 3eff2277bb4dd6a53eb363e3ae37fea1ada44042; Bazel shutdown 0

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Summary by cubic

Assembles the real Forge process around ForgeApp with loopback QUIC, credential custody, readiness publication, cancellation, and ordered shutdown. Replaces the no-op backend entry point with explicit configuration and typed process exit outcomes.

• forge_runtime is the single boundary that turns application, storage, custody, and transport owners into one process.
• Readiness publishes only after storage migration and loopback binding complete.
• Adds 13 runtime tests covering configuration, readiness identity, service/drain failures, cleanup precedence, and restart behavior.
• Moves rustls-pki-types to a regular dependency and enables tokio's signal feature.
• Preserves the prior Statig dependency graph with a repinned Crate-Universe checksum.

^{Written for commit 3eff227. Summary will update on new commits.}