refactor(symbols): replace get_name_for_subnet match with static NAMES table

[boskodev790]

## Summary

- `get_name_for_subnet` was a 439-arm `match` where every arm called `b"name".to_vec()`, allocating a fresh `Vec<u8>` on every call even though the subnet names are compile-time constants; replace it with a parallel `pub static NAMES: [&[u8]; 439]` lookup table that mirrors the existing `SYMBOLS` array by `netuid`
- `get_name_for_subnet` now prefers a non-empty `SubnetIdentitiesV3::subnet_name` (unchanged), falls back to `NAMES[netuid]` as `&'static [u8]`, returns `b"unknown"` for out-of-range netuids, and pays exactly one `.to_vec()` at the return boundary instead of one per match arm
- All 439 name-to-netuid mappings are preserved byte-for-byte, including the deliberate `b"unknown"` sentinel slots at indices 55 and 60 and the single multi-word entry `b"red team"` at index 61 — cross-checked programmatically against the original match before and after the edit
- Add `pallets/subtensor/src/tests/symbols.rs` with six `#[test]` cases covering first / last / space-containing / sentinel entries, out-of-range fallback, stored-identity-wins-over-table, empty-identity-falls-through-to-table, and a length invariant between `NAMES` and `SYMBOLS` so a future edit to one without the other fails fast in CI
- External contract is unchanged — the function signature still returns `Vec<u8>` and all three callsites (`rpc_info::dynamic_info::subnet_name`, `rpc_info::metagraph::name` and its helper) continue to receive the same bytes

## Related Issues

Closes #1520.

## Type of Change

- [ ] Bug fix
- [ ] New feature
- [x] Refactor
- [ ] Documentation
- [ ] Other (describe below)

## Testing

- [x] Tests added — `tests/symbols.rs` pins the six behavioural contracts listed above
- [x] Refactor is behaviour-preserving — verified programmatically by extracting every `idx => b"name".to_vec()` pair from the original `main`-branch file and asserting 1-for-1 equality with the new `NAMES` array entries (439 of 439 matched)
- [ ] `cargo check` / `cargo test --package pallet-subtensor` / `cargo clippy` — not run locally because the Rust toolchain and the Substrate dependency graph are not installed in this environment; the diff is structurally identical to the sibling `get_symbol_for_subnet` pattern already present in the same file

## Checklist

- [x] Code follows project style guidelines (mirrors `get_symbol_for_subnet` pattern)
- [x] Self-review completed
- [x] No functional change — all three production callers receive the same bytes