feat: add pitch pallet - #75

Summary

Adds fc-pallet-pitch, a first-cut FRAME pallet for ephemeral, consent-gated economic cells keyed by H3 cell references.

The pallet implements the minimal consensus surface from the Decent/Kreivo pitch spec:

• claim, amend, join, dissolve, and reap time-boxed pitches;
• index public raw-cell pitches by H3 u64 for cheap lookup;
• keep committed/private pitches out of the raw-cell index;
• support disclosure tiers: Public, Unlisted, Gated, Sealed;
• support pluggable membership verification via a MembershipVerifier trait;
• record selective disclosure grants without putting disclosed secrets on-chain;
• derive deterministic per-pitch sovereign accounts from the pallet ID.

This intentionally does not wire payment/fee routing or VOS/Noir verification yet. Those are runtime integration decisions; this PR keeps the pallet small enough to review and gives us the base types/storage/lifecycle to discuss.

Verification

• cargo test -p fc-pallet-pitch
• cargo check --workspace