Re-sequence new-format timeslice patterns onto snapshots

[nick-gorman]

Adds the translator-side timeslice module. It maps each templated timeslice window pattern onto the model's actual snapshots, producing a timeslice_snapshots table (timeslice_id, investment_periods, snapshots) that later PRs use to apply per-timeslice link limits and to scope the temporal custom constraints.

Each model financial year is assigned a reference year with the same construct_reference_year_mapping call the trace pipeline makes, so timeslice windows and demand/VRE traces always come from the same weather years — the binding interaction between hot-day limits and hot-day demand survives any configured cycle. The year before the first model year also gets a pattern (the cycle's last reference year, cycle-consistent) because winter windows run April–October and must cover the first model year's July–September snapshots.

• Month-day boundaries of 29 February clamp to 28 February in non-leap model years. The wrap decision for exclusive ends compares month-day strings rather than the placed dates, so clamping a one-day 02-28..02-29 window collapses it to empty instead of stretching it out a year.
• A configured cycle requesting reference years the timeslices table has no patterns for raises rather than silently never binding.

Dark: nothing imports the module yet (the new-format path is wired in a later PR), so no production behaviour changes — it merges on its unit tests alone.

src/ispypsa/translator/
└── timeslices.py                        → map timeslice patterns onto model snapshots
tests/test_translator/
└── test_timeslice_snapshots.py          → module tests (10)

[codecov]

Codecov Report

❌ Patch coverage is 96.82540% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/ispypsa/translator/timeslices.py	96.82%	1 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
src/ispypsa/translator/timeslices.py	96.82% <96.82%> (ø)

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