feat: native vector search support

[rebasedming]

What

Implements paradedb/paradedb#5685: native vector search support with no neighbor gem dependency.

• vector(n) ActiveModel type with t.vector :embedding, limit: n migration DSL and schema-dumper round-trip
• Model.nearest(:embedding, vec).limit(k) — orders by the pgvector operator, defaults the metric from the index opclass, and auto-adds key_field @@@ pdb.all() when the relation has no ParadeDB predicate
• Arel builder/predication distance helpers (<->, <=>, <#>)
• Vector fields in the bm25 index DSL via embedding: { metric: :cosine }, emitted as opclasses and round-tripped by the schema dumper
• hybrid_rrf example migrated off neighbor; new vector_search example; vector integration tests capability-gated until a release ships vector-in-bm25

Why

ParadeDB indexes pgvector columns inside the bm25 index, so users should get the full query surface from this gem alone.

Tests

Rails 8.1 and 7.2: 387 unit / 289 integration examples, 0 failures (2 pending vs 0.24.2 via capability gate; 0 pending vs a dev pg_search 0.25.0 where EXPLAIN confirms Top-K pushdown). rubocop/api-coverage/gem-install smoke all clean.

[codecov]

Codecov Report

❌ Patch coverage is 98.34711% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.83%. Comparing base (afe980b) to head (d47a2ae).

Files with missing lines	Patch %	Lines
lib/parade_db/vector.rb	96.96%	2 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           paradedb-am     #130      +/-   ##
===============================================
+ Coverage        90.43%   90.83%   +0.40%     
===============================================
  Files               20       21       +1     
  Lines             2425     2542     +117     
  Branches           779      811      +32     
===============================================
+ Hits              2193     2309     +116     
- Misses             232      233       +1     

Flag	Coverage Δ
rails-paradedb	90.83% <98.34%> (+0.40%)	⬆️
rb3.2	91.09% <98.33%> (+0.38%)	⬆️
rb3.3	91.09% <98.33%> (+0.38%)	⬆️
rb3.4	90.83% <98.34%> (+0.40%)	⬆️
rb4.0	82.61% <95.04%> (+0.63%)	⬆️
rl7.2	90.83% <98.34%> (+0.40%)	⬆️
rl8.1	82.61% <95.04%> (+0.63%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
lib/parade_db.rb	98.14% <100.00%> (+0.03%)	⬆️
lib/parade_db/arel/builder.rb	94.42% <100.00%> (+0.23%)	⬆️
lib/parade_db/arel/predications.rb	97.77% <100.00%> (+0.13%)	⬆️
lib/parade_db/index.rb	90.50% <100.00%> (+0.44%)	⬆️
lib/parade_db/migration_helpers.rb	84.91% <100.00%> (+0.20%)	⬆️
lib/parade_db/model.rb	84.31% <100.00%> (+1.09%)	⬆️
lib/parade_db/search_methods.rb	94.23% <100.00%> (+0.10%)	⬆️
lib/parade_db/vector.rb	96.96% <96.96%> (ø)

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[philippemnoel]

Suggested change

	puts "\nCombining ParadeDB BM25 + native vector distance in one CTE-based query"
	puts "\nCombining ParadeDB BM25 + vector distance in one CTE-based query"

Wdyt?

[philippemnoel]

perhaps vector search should be higher? Like right below Quickstart maybe, on all ORMs?

[philippemnoel]

see my comment on EF Core PR