Fix UTF-8 encoding corruption in R scripts reading JSON from stdin

[Copilot]

Description

UTF-8 characters in series labels (e.g., '≥30.78') were corrupted to '��30.78' in R script outputs. R was defaulting to system encoding rather than UTF-8 when reading stdin.

Changes

Fixed four R scripts that read JSON data containing potentially non-ASCII labels:

• R/src/survival.R - survival analysis
• R/src/cuminc.R - cumulative incidence analysis
• R/src/regression.R - regression analysis
• R/src/density.R - density calculations

Each changed from:

con <- file("stdin", "r")
json <- readLines(con)

To:

con <- file("stdin", "r", encoding="UTF-8")
json <- readLines(con)

The readLines() call inherits encoding from the connection, so no additional parameter needed.

Checklist

Check each task that has been performed or verified to be not applicable.

• Tests: Added and/or passed unit and integration tests, or N/A
• Todos: Commented or documented, or N/A
• Notable Changes: updated release.txt, prefixed a commit message with "fix:" or "feat:", added to an internal tracking document, or N/A
• Rust: Checked to see whether Rust needs to be re-compiled because of this PR, or N/A

[xzhou82]

this fix doesn't work
test link still shows