Implement a mapped variant status taxonomy

[bencap]

Replace the binary mapped/failed outcome with explicit and consistent status strings. Proposed minimal taxonomy based on what we've seen so far:

• mapped_clean — no mismatches, gaps, or ambiguity flags near the variant
• mapped_with_mismatch — variant maps onto or adjacent to a mismatched column
• mapped_near_gap — variant within N residues of a gap boundary
• failed_outside_region — variant outside the aligned portion of the query

Different status codes can route to different curator triage requirements, and the mapping table can aggregate counts per status to surface patterns across a target's variants.