Add storage usage introspection helper for crowdfund_vault (#863)

[xtrial01]

Closes #863

Adds lightweight storage usage introspection helpers to crowdfund_vault.

Changes:

• ProjectStorageSummary struct (storage.rs)
• get_project_storage_summary() read-only query (lib.rs)
• get_refund_receipt_count() helper (lib.rs)
• 3 unit tests covering nonexistent project, existing project,
  and total_projects accuracy (test.rs)
• Maintainer docs in SMART_CONTRACTS.md with usage examples

All read-only — no storage writes, no added rent cost.
Build passing, all tests green.

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[Cedarich]

@xtrial01

[xtrial01]

@xtrial01

Working on it

[xtrial01]

@xtrial01

recheck boss

[Cedarich]

@xtrial01 please fix failing workflow

[xtrial01]

@xtrial01 please fix failing workflow

Fixed - resolved the merge conflict and formatting issue, CI re-running now.

[xtrial01]

@xtrial01 please fix failing workflow

Fixed - resolved the merge conflict and formatting issue, CI re-running now.

pls recheck now

[Cedarich]

@xtrial01