EQUIL - IMPROVEMENT - Scope custom-Jacobian knobs to jac_custom_power_*

[logan-nc]

Closes #288.

Problem

In [Equilibrium], the Jacobian coordinate system is chosen with jac_type. For named systems (hamada, pest, boozer, equal_arc, park) the constructor forces a fixed set of exponents, yet the raw power_bp/power_b/power_r/power_rc were exposed as top-level keys that look universal but only mattered when jac_type = "other". Listing them (as 0) in every example implied false relevance, and setting one on a named-type run was silently ignored.

Changes

Custom-Jacobian knobs (src/Equilibrium/EquilibriumTypes.jl)

• Rename jac_type = "other" → jac_type = "custom" (clearer intent; no shim — no stable API pre-2.0.0).
• Add user-facing jac_custom_power_bp/b/r/rc (default 0), read only at construction when jac_type = "custom", then copied into the internal power_*. The existing "recognize a named type from the exponents" normalization is retained (fast paths still taken).
• Named types are unchanged: they force power_* and ignore the custom knobs.
• Downstream is untouched — FieldLineDerivParams and every Jacobian formula (DirectEquilibrium.jl, DirectEquilibriumArcLength.jl, InverseEquilibrium.jl) keep reading the internal power_* fields as the single source of truth. The four now derived-only constructor slots are marked _.

Deprecate power_* as TOML input keys (reusing the centralized mechanism)

• Generalize _drop_deprecated_ffs_keys!(table) → _drop_deprecated_keys!(table, deprecated_keys, section) in src/GeneralizedPerturbedEquilibrium.jl.
• Add _DEPRECATED_EQUIL_KEYS = ("power_bp", "power_b", "power_r", "power_rc"), applied on both the gpec.toml (build_inputs_from_toml) and h5-replay (src/Rerun.jl) input paths. A stale power_* key now warns loudly and is dropped instead of silently overriding the coordinate system.
• Fix a latent bug: _DEPRECATED_FFS_KEYS = ("mer_flag") was a string (not a 1-tuple), so for k in … iterated its characters and the FFS deprecation never fired. Now ("mer_flag",).

Docs & examples

• Update the EquilibriumConfig docstring (surfaced via @autodocs) and docs/src/conventions.md to document custom and the jac_custom_power_* fields with J = Bp^bp · B^b / (R^r · rfac^rc).
• Standardize the jac_type option-list comment to (hamada, pest, boozer, equal_arc, park, custom) and remove the now-inactive power_* lines from the example/test gpec.toml files (safe — named types force the exponents regardless of file contents).

Verification

• Unit checks: custom copies knobs → internal exponents and normalizes to the named fast path when they match; a non-standard set stays custom with manual exponents; named types force exponents and ignore custom knobs; the deprecation warns + drops and the config still builds from jac_type; all 19 example/test gpec.toml files parse.
• Regression harness (diiid_n1, solovev_n1, solovev_multi_n, local vs develop): every quantity 0.0e+00 diff — the change is numerically inert.

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