feat: add per-holder accrual snapshot query

[Glam26]

Here's a summary of all changes made across 3 files (258 additions, 8 deletions):
──────────────────────────────────────────────────────────────────────

1. src/lib.rs — New get_holder_accrued_unclaimed function (+113 lines)
   Event constant ( EVENT_ACC_SNAP ):

• New debug tracing event emitted on every call to the new query, giving integrators a computation breakdown in simulation responses.
  New public function get_holder_accrued_unclaimed(env, issuer, namespace, token, holder) -> i128 :
• Pure read-only — no state mutation, no auth required
• Returns 0 immediately for blacklisted holders
• Computes total unclaimed dividends by:

1. Loading the holder's HolderAccrualState (frozen accrued_owed + last_settled_idx )
2. Computing additional accrual from last_settled_idx → matured end via find_matured_claim_end_idx + compute_holder_payout_for_range
3. Adding any AccrualAnchor.claimable_sum (compressed share-schedule entries)
4. Emitting EVENT_ACC_SNAP with the full breakdown

• Gas bounded per-holder — scales with share-schedule entries (naturally bounded by checkpoint compression at 1,000 entries)
• Inserted after get_claimable_chunk , before the Time-delayed claim configuration section
  ──────────────────────────────────────────────────────────────────────

2. src/test_accrual_ledger.rs — 8 new tests (+144 lines)
   ┌───────────────────────────────────────┬──────────────────────────────┐
   │ Test │ What it checks │
   ├───────────────────────────────────────┼──────────────────────────────┤
   │ accrued_unclaimed_returns_zero_for_bl │ Fast-path guard works │
   │ acklisted_holder │ │
   │ accrued_unclaimed_matches_claimable_f │ 50% share → accrued == │
   │ or_single_period │ claimable == 50k │
   │ accrued_unclaimed_matches_claimable_a │ 50% → 25% share: accrued == │
   │ fter_share_change │ claimable == 75k │
   │ accrued_unclaimed_returns_zero_for_ho │ Never had share → 0 │
   │ lder_with_no_share │ │
   │ accrued_unclaimed_respects_partial_cl │ Claimed 1 of 2 periods → │
   │ aim │ remaining = 50k │
   │ accrued_unclaimed_after_full_claim_re │ All periods claimed → 0 │
   │ turns_zero │ │
   │ accrued_unclaimed_zero_share_does_not │ Had 50% share then set to 0 │
   │ _erase_past_accrual │ → past accrual (50k) │
   │ │ preserved │
   │ accrued_unclaimed_matches_claimable_a │ Combination: share changes + │
   │ fter_partial_claim_with_share_change │ partial claim → matches │
   └───────────────────────────────────────┴──────────────────────────────┘
   Every test asserts accrued == claimable when both are meaningful.
   ──────────────────────────────────────────────────────────────────────
3. tools/storage_layout_schema.rs — Pre-existing drift fix (-8, +1 lines)

• Added: DataKey2::FaucetSeedCount(OfferingId) — was missing from the registry
• Removed: 8 stale variants no longer in the DataKey2 enum ( EmitV2Compat , GlobalFreezeReason , AdminRotationDelay , JurisdictionGracePeriod , JurisdictionMigration , RedemptionRequest , RedemptionFeeConfig , LockupSchedule )

closes #529

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