Add value, weight to gen_psa_samp parameterization choices

[jurjoroa]

Goal

"value, weight" is documented as the only valid parameterization for dists == "bootstrap" but is missing from the parameterization_types choices vector, so match.arg(several.ok = TRUE) silently drops it and shifts every later element down one position. A subsequent distribution is then reinterpreted under the wrong parameterization with no warning, or the shift lands on NA and crashes downstream comparisons.

Scope

• Add "value, weight" to the parameterization_types default at R/gen_psa_samp.R:134
• Add a regression test asserting index alignment when bootstrap is not the last entry in dists
• Add a test covering the NA crash path at R/gen_psa_samp.R:142
• Record the behavior change in NEWS.md — samples that silently used a shifted parameterization will now differ
• Re-verify the audit repro on-branch before applying the proposed diff — this fix was reasoned from root cause, not re-run to green

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Version: #172