Add Salim+18 modified Calzetti attenuation law (δ, B) with end-to-end ProSpect integration

[Copilot]

This PR introduces support for the Salim et al. (2018) modified Calzetti attenuation law so ProSpect can model flexible dust-screen curves via slope tilt (δ) and UV bump strength (B), instead of assuming a single fixed screen law.

• Dust law implementation (R/dust.R)

  • Added Salim18_screen(...) for modified Calzetti attenuation.
  • Added screen_atten(...) dispatch wrapper (dust_law = "CF" | "Salim18").
  • Extended CF_screen_atten(...) and atten_emit(...) to accept law-selection and Salim+18 parameters (delta, B, Rv) while preserving existing CF defaults.

• High-level API wiring (R/SFH.R, R/ProSpect.R)

  • Added new screen-law controls to SFHfunc, SFHburst, and ProSpectSED:
    • dust_law = "CF"
    • delta_screen = 0
    • B_screen = 0
    • Rv_screen = 4.05
  • Replaced direct CF_screen(...) usage in screen paths with screen_atten(...).
  • Propagated the same controls into AGN screen attenuation paths via atten_emit(...).

• Documentation updates (man/*.Rd)

  • Documented new functions and parameters in:
    • man/dust.Rd
    • man/SFH.Rd
    • man/ProSpectSED.Rd
  • Clarified Salim18 behavior and parameter semantics alongside existing CF options.

• Targeted regression/behavior coverage

  • Added tests/test_salim18_dust.R validating:
    • CF backward-compatibility through the wrapper.
    • Salim18 attenuation normalization behavior.
    • Eb -> B fallback handling in Salim18 mode.

spec <- ProSpectSED(
  dust_law = "Salim18",
  tau_screen = 0.4,
  delta_screen = -0.3,
  B_screen = 1.0,
  Rv_screen = 4.05
)

[asgr]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved the merge conflicts and merged the latest origin/master into this PR branch. The conflict in R/dust.R is fixed in commit 8d6c81d.