fix(core): type absent records as Error::NotFound (was InvalidData)

[Nic-dorman]

Problem

data::Error has no not-found variant, so a well-formed address with nothing stored at it surfaces as Error::InvalidData — an error class that means the caller sent something malformed. The absent-record signal already exists (chunk_get returns Ok(None)); it's the conversion sites that stringify it away:

• data_map_fetch / data_map_fetch_from_closest_peers → InvalidData("DataMap chunk not found at …")
• data_download's reconstruction miss → InvalidData("Missing chunk … required for data reconstruction")

Downstream this misleads real users: ant-ffi maps InvalidData → InvalidInput, so the mobile SDKs show "invalid input" for a mistyped or absent address and can't offer "not found — check the address" UX. (Mobile tracker: V2-650; sibling of #152 — same lossy-stringification family.)

Change

• New Error::NotFound(String) variant, documented against InvalidData (absent record vs retrieved-but-malformed/integrity-failing content).
• The three definitively-absent conversion sites now return it (messages unchanged). The genuine integrity checks (mismatched chunk address / content hash in chunk.rs) correctly remain InvalidData.
• classify_error: NotFound → Outcome::ApplicationError — the peers answered over a working link, so absence must not push the adaptive limiter down. (The Ok(None) → Timeout load-shedding signal inside chunk_get_observed is fed pre-conversion via chunk_get_outcome and is unaffected.)
• Doc updates on the three public fns' # Errors sections.

Tests

• classify_error_covers_all_variants + the compile-time exhaustiveness guard extended.
• Display test for the new variant.
• cargo fmt --check ✓ · cargo clippy --all-targets --all-features -- -D warnings ✓ · cargo test -p ant-core --lib 411 passed · cargo test -p ant-cli 18 passed.

Consumer

Once this ships in an ant-core release, ant-ffi adds a one-arm From mapping to its existing ClientError::NotFound and mobile apps can finally distinguish not-found from caller error.

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