fix(weather): wire forecast cache + weight-aware throttle + variable trim

[zax0rz]

Summary

Fixes #64. Three compounding issues cause Open-Meteo free-tier rate-limit exhaustion during backtests: uncached forecasts re-fetched on every research() call, a politeness throttle blind to weighted call costs, and over-fetching 18 variables when only 3 are consumed by the pairs join.

This PR wires the existing Phase 20 forecast cache into _fetch_open_meteo_range (research.py), adds weight-aware client-side date chunking for Previous Runs requests exceeding 14 days, and trims the variable set on the research() path to the 3 columns the pairs join actually uses.

Test plan

• test_open_meteo_cache_wiring.py (247 lines) — cache hit, cache miss, partial cache, stale cache, NaT-to-None cleanup before Parquet, duplicate-row guard on non-contiguous cache miss
• test_open_meteo_variables_param.py (152 lines) — variable subset validation, unknown variable rejection, endpoint _previous_day1 suffix handling with subset, full-set passthrough
• test_open_meteo_window_chunking.py (143 lines) — single-chunk passthrough, multi-chunk boundaries, weight-aware delay calculation, Single Runs bypasses chunking
• All upstream tests pass (ruff + ruff-format + pytest pre-push clean)
• Full suite confirmation in upstream CI

Cross-SDK Sync (see .planning/CROSS-SDK-SYNC.md)

python_only: true — all changes are in the Python weather fetcher and core research module; no parity-trigger paths touched

[helloiamvu]

Codex review (gpt-5.5, medium reasoning) — 1 blocking finding (P2):

• [P2] Partial-month forecast cache treated as complete — packages/core/src/mostlyright/research.py:1431-1432. When the first request for a month covers only a subrange (e.g. 2024-06-01..2024-06-02), only that subrange is written to the monthly partition, but a later request for a different June window sees hit is not None and skips the network fetch — silently dropping Open-Meteo forecasts for the uncached days in that month. Needs coverage metadata, or the writer must populate full-month partitions before the read path treats a partition as complete.

CI is green and the diff is solid otherwise; this is the one correctness issue to resolve before merge. (Reviewed as part of the 2026-06-06 autonomous triage run — this is feature-scope, recommend landing in a 1.6.0 minor rather than the v1.5.3 patch.)

[helloiamvu]

Incorporated into the v1.6.0 release via the integration branch merged-vision → main (#71, merged as 4c0bd19). The branch content was merged plus a follow-up fix for the Codex P2 (full-month cache partitions) and three further P2s found during integration review (chunk-concat attrs, Single-Runs delay scaling, seamless chunking, weather>=1.6.0 floor). Published to PyPI as mostlyrightmd/-weather/-markets 1.6.0. Thanks @zax0rz!