Apply the new-format custom constraints in pypsa_build

[nick-gorman]

The new-format custom constraints from the translator (#139) are scoped by
investment period and timeslice, e.g. a flow limit that only applies at QLD
peak demand in 2030. The existing _add_custom_constraints sums every term
over every snapshot, so it can't express that. This change adds
_add_custom_constraints_with_temporal_scope, which turns each RHS row into
one linopy constraint at the snapshots its investment period and timeslice
select.

It isn't wired into build_pypsa_network yet; that comes with the final
wiring PR, so there's no change in behaviour.

src/ispypsa/
├── pypsa_build/custom_constraints.py        ← new temporal-scope path
├── translator/timeslices.py                 ← no-snapshot warning now per investment period
└── validation/schemas/
    ├── ispypsa_tables/                      ← existing schemas, moved
    └── pypsa_friendly_tables/               ← new: contracts for this step's inputs
tests/test_model/test_custom_constraints_with_temporal_scope.py
CLAUDE.md                                    ← "investment period", never bare "period"

Design choices worth a look:

• Blank timeslice is a fallback, per constraint. It covers the snapshots
  in its investment period that none of the same constraint's named
  timeslices cover.
• Blank investment_period matches only blank. An LHS term joins only the
  RHS rows with its exact investment_period, not every investment period. The
  translator resolves all dated inputs to explicit investment periods, so
  blank only appears on the expansion limits.
• Load terms are data, not variables. coefficient × p_set is added as a
  per-snapshot constant, which linopy moves to the RHS.
• Storage p is net dispatch (p_dispatch − p_store), matching PLEXOS's
  paired battery Generation/Load coefficients.
• Missing components aren't skipped. Unsupported (component, attribute)
  terms and constraints with no variable terms raise, rather than silently
  weakening the constraint.
• Schema split. The schema folder is now ispypsa_tables (translator
  input) and pypsa_friendly_tables (pypsa_build input). Only the three
  tables this step reads are declared so far. Nothing loads the schemas yet,
  so the move is paths only.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/ispypsa/pypsa_build/custom_constraints.py	93.49% <100.00%> (+13.49%)	⬆️
src/ispypsa/templater/existing_planned.py	100.00% <ø> (ø)
src/ispypsa/templater/new_entrants.py	100.00% <ø> (ø)
src/ispypsa/translator/timeslices.py	100.00% <100.00%> (ø)

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