feat: add baseline package tests and private-safe security audit - #5

In @.github/workflows/security-audit.yml:
- Around line 25-38: The workflow currently downloads and runs external binaries
without integrity checks; update the "Install gitleaks" and "Validate workflows"
steps to fetch and verify release checksums (or signatures) before execution:
for the gitleaks step (refer to the "Install gitleaks" and subsequent
"./gitleaks detect" usage) download the official checksum (e.g., SHA256SUMS or
.sha256) for v8.24.2 from the gitleaks release, verify the tarball with
sha256sum (or gpg verify if a signature is provided) and fail the job if
verification fails, then extract and chmod only after verification; for
actionlint (the script piped from rhysd/actionlint and the "./actionlint"
invocation in "Validate workflows") either download a released artifact and its
checksum/signature and verify before running or fetch the script plus the
published checksum/signature and validate the downloaded binary/version matches
the checksum, aborting on mismatch.

In `@R/aFIPC.R`:
- Around line 200-215: The while loop that repeatedly calls mirt::mirt to create
oldFormModel can loop forever if model fitting keeps failing; modify the code
around oldFormModel (the try + while (!exists('oldFormModel')) block that calls
mirt::mirt with data = oldformYDataK and itemtype = itemtype) to implement a
bounded retry: introduce a max_retries counter (e.g., 5–10), increment an
attempt counter on each failure, catch and store the error from mirt::mirt, add
a short backoff (Sys.sleep) between attempts, and after exceeding max_retries
stop retrying and handle the failure (log or return a clear error/NA) instead of
spinning; apply the same change to the analogous block that creates the new form
model.
- Around line 137-165: The code wraps mirt::mirt calls in try() (e.g. the
assignment to oldFormModel via mirt::mirt) but never checks whether the call
actually returned a valid model before later accessing slots like `@OptimInfo`;
update the try usage to capture the result (e.g. store try(...) into a variable
or use tryCatch), verify the object is a valid mirt model (or non-NULL) after
the call, and if it failed either stop with a clear error or branch to a
fallback path (same fix for oldFormModel and the corresponding newFormModel
block around the second mirt call); reference oldFormModel, mirt::mirt, and
`@OptimInfo` when making the check and handling failure.
- Around line 590-597: The code unconditionally removes parameters matching
"^ak" from IPDParmNames, which incorrectly excludes ak* for nominal items;
update the logic around the IPDParmNames filtering so that ak parameters are
only removed when the item's type is not "nominal" (i.e., check the itemtype or
a variable indicating nominal vs non-nominal before applying the grep("^ak",
IPDParmNames) removal), keeping the existing removals for "^MEAN", "^COV", and
"^d0$" unchanged and only conditionally including grep("^ak", IPDParmNames) in
the subtraction when itemtype != "nominal".

In `@R/aFIPC.R`:
- Around line 101-108: The function checkoldformBILOGprior calls an undefined
readinteger(), causing a runtime error; change the error branch to recursively
call checkoldformBILOGprior() (matching the pattern used in checkCorrect and
checknewformBILOGprior) so invalid input re-prompts correctly and returns the
validated value from checkoldformBILOGprior().