Add repos-v0 registry contract crate

[LandynDev]

Part of repo-registration phase 2a (task 2a-2). Implements the repos-v0 ink! 5 contract per gt-utils spec specs/repo-registry-contract.md, mirroring the issues-v0 crate layout.

What it does

• Repository registry keyed by GitHub numeric id: register / deregister / transfer_ownership / update_full_name (rate-limited rename follow)
• Dynamic registration fee: Bittensor-style price curve (halvings by shift + Q32 fractional multiply, exact port of subtensor misc.rs), pulled from the caller's stake via CallerTransferStakeV1 and recycled via RecycleAlphaV1; delta-verified so a silent shortfall reverts
• Count-based prune of the least-supported non-immune repo when full (all-immune registers reject before any fee is taken); select_victim is the single 2b swap point for stake-weighted selection
• Per-repo hyperparams as key-value with owner-settable bounds (26 seeded keys), label multipliers, branch patterns
• Whitelist-gated validator baskets (set_basket, weights sum 65535, entries validated against active registry)
• Owner admin: pause, set_code_hash upgrade path, config with price_last re-clamp, force_deregister escape hatch
• Launch constants per spec: MAX_REPOS=32, 30d immunity (216000 blocks), 500-alpha floor
• Storage layout byte-offsets documented in lib.rs for the python childstate reader; all events carry github_id + full_name for the mirror reconciler

Spec deviations (flagging for review)

1. register(github_id, full_name, fee_hotkey) — third param added; stake is keyed by hotkey so the contract cannot locate the caller's alpha without it
2. Params/labels stored as Mapping<u64, Vec<(k, v)>> per repo instead of tuple-keyed cells — tuple keys are unenumerable for the pure-storage python reader
3. Hard caps not in spec: MAX_REPOS_HARD=256, MAX_VOTERS=64, MAX_PARAM_KEYS=64
4. Pruned/deregistered repos keep their Repo record with active=false; stale basket entries fail re-validation on next set_basket

Verification

• 71/71 unit tests, clippy 0 warnings (cargo contract build not run — cargo-contract not installed locally)
• Price-curve golden vectors (PRICE_GOLDEN, 14 cases + DECAY_FACTOR_GOLDEN) generated from an independent integer-exact python replica; these seed the 2a-3 python client port
• Contract-security checklist reviewed: owner/repo-owner/voter authorization on every mutating message, saturating arithmetic throughout, chain-ext return values checked, bounded iteration (repos <= 256, voters <= 64), state cleanup on removal verified by tests