Fix libxc 6.2.x support; refresh v7.1 headers from released 7.1.2 (v0.1.3)

[ajz34]

Summary

Follow-up to the FFI correctness revalidation against real 6.2.2 / 7.0.0 / 7.1.2 shared libraries. Four commits, targeting v0.1.3:

1. Fix libxc 6.2.x support

• Cargo feature isolation: the workspace libxc-ffi dependency now sets default-features = false, so api-version features are selected exclusively through the libxc crate's features. Previously libxc-ffi's own defaults (api-v7_0 + dynamic_loading) were always unified in, and --no-default-features --features api-v6_2,dynamic_loading silently compiled the v7.0 struct layout of xc_func_type — reading garbage when a v6.2.2 shared library is loaded at runtime.
• mGGA tau handling for libxc < 7.0: XC_FLAGS_NEEDS_TAU only exists in v7.0+. libxc 6.2.x reads the tau input for every meta-GGA functional, and xc_mgga_sanity_check aborts the process (exit(1)) when a tau-family output of a requested derivative order is NULL. The wrapper now:
  • falls back to the functional family in needs_tau() for < 7 libraries (mirrors pylibxc 6.2.2),
  • substitutes zeroed scratch for null lapl/tau inputs,
  • allocates zeroed scratch (MggaExtraPtrs) for tau-family outputs missing from the layout or output pointer map. CUDA paths pass an empty map: 6.2.x has no GPU support, and v7.0+ gates tau on the flag.

2. Refresh v7.1 headers from the released 7.1.2 tag

libxc v7.1 is now released upstream (7.1.0–7.1.2); the bundled v7.1 headers were snapshotted from the devel branch before the release:

• xc_funcs_v7.1.h (XcFuncId v7_1 regenerated): add T_HLE17, LDA_C_BJ89, GGA_X_LLP, LDA_C_LP96_B, LDA_K_LP96_B, LDA_C_RPAF, HYB_MGGA_XC_COACH, MGGA_X_SREGTM_V1/V2/V3; remove MS2BS, MVSB, MVSBS, OPB3LYP (never shipped in a released v7.1). All shared IDs keep their numbers.
• xc.h: differences vs 7.1.2 are comment-only (verified identical after comment stripping), so FFI bindings are unaffected; refreshed for provenance. xc_version.h macros corrected 7.0.0 → 7.1.2.
• xc_funcs_v6.2.h turned out not to be the 6.2.2 tag file either: it contained 44 post-6.2.2 functional IDs, a typo'd LDA_C_1D_CSS (real name LDA_C_1D_CSC), LDA_XC_TH_FL (real name GGA_XC_TH_FL), and was missing MGGA_X_MK00. Replaced with the 6.2.2 tag file and regenerated. Note 7.1.2 itself renames 1D_CSC → 1D_CSS and GGA_XC_TH_FL → LDA_XC_TH_FL, so those variant names are version-dependent by design.
• xc_funcs_v7.0.h already matched the 7.0.0 tag exactly (regenerating is byte-identical).

Docs updated to reflect that v7.1 is a released line: readme, versioning_xc.md, CLAUDE.md, cuda-testing skill, CHANGELOG.

3. Version bump to v0.1.3 (2026-09-08)

4. CI fixes

• test-v7.0 workflow: select dynamic_loading explicitly — it silently relied on libxc-ffi's default features before the isolation fix, and failed at the link stage once those stopped being unified in — and pin conda-forge libxc to 7.0.* (conda-forge's default is now 7.1.2, which does not pair with api-v7_0 bindings by design).
• cargo fmt --all: current rustfmt reflows long trailing comments, flagging files untouched since May; includes two genuine over-width line splits in compute_cpu.rs from this PR.

API impact

• No public function signatures change; all new internals are pub(crate).
• Behavioral: needs_tau() now returns true for every mGGA/hyb-mGGA when the loaded library is v6.2.x (matches pylibxc 6.2.2); unchanged for v7.x.
• XcFuncId variant sets change under api-v7_1 (+10/−4) and api-v6_2 (+3/−45). The removed variants never worked against real released libraries, so this is a correctness fix.
• With default-features = false, building without any api feature now fails loudly (unresolved imports) instead of silently compiling v7.0 defaults.

Validation

cargo test -p libxc against locally built shared libraries, all passing (mGGA compute tests included):

libxc	variant	features
6.2.2	CPU	--no-default-features --features api-v6_2,dynamic_loading
7.0.0	CPU	default (api-v7_0)
7.1.2	CPU	api-v7_1
7.0.0	CUDA	cuda
7.1.2	CUDA	cuda,api-v7_1

Feature isolation verified via cargo tree -f "{p} [{f}]": libxc-ffi resolves to [api-v6_2, dynamic_loading] only under the v6.2 selection.