Derive the backing family from the chain id (subnet alpha, PR1)

[LandynDev]

Derive a leg's backing family from its chain id so subnet alpha tokens (sn<N>) settle in the TAO purse with no new accounts, instructions or config.

• backing::family(chain): sn<digits> -> tao, else the chain itself. An alpha shares subtensor's account space, so the existing vote_slash(.., user: AccountId, ..) already pays its holder.
• collateral_leg_bind replaces collateral_leg_amount: a leg that IS the backing asset binds Exact(amount) (today's equality check, byte-identical for every sol/tao pair, pinned by a differential test); a leg that only settles in the family binds Declared — bounded by swap_bounds, understatement policed off-chain (spec §2, §5).
• Exact legs are matched before family legs, so sn7<->tao keeps the free on-chain check in both directions. This is an amendment to the spec's §3 snippet, which checked the family first.
• collateral_leg_user_addr and declarable_bit use the same lookup.
• fulfillment_grace_secs: every TAO-family chain takes TAO's 180s window. CANCEL_REASON_ALPHA_TRANSFER_DISABLED = 6 for a subnet that disables transfers mid-swap.

Tests: cargo test -p allways_swap_manager --lib (54).

Deploy: one program redeploy + testnet era bump. Adding another subnet later is a Python registry row, no contract change.

Spec: SPEC_SUBNET_TOKENS.md §2–§3. Stack: PR1 (this) -> PR2 alpha assets -> das registry -> PR3 gate.