Handle ambiguous effects more reliably and emit diagnostics for these

[lionel-]

Progress towards #1338

• a library() in an if/else branch no longer leaks its attach effect into the sibling branch during scan.

• A lazy body (function, reactive()) defined inside a branch still sees that branch's attach at its own definition point, even though the attach doesn't survive past the branch's join.

• An attach or binding conditional in a branch/loop that might not run now logs an effect ambiguity diagnostic:

if (cond) {
  local <- identity
}
local({
  y <- 1
})

if (cond) library(shiny)
reactive({
  x <- 1
})

• New EffectAmbiguity diagnostic (replacing LazyShadowAmbiguity/ConditionalShadowAmbiguity) flags three cases where an effect decision depends on a runtime condition: lazy-crossed shadowing, conditional local shadowing, and now conditional attaches too.

These diagnostics are currently only logged but will be surfaced in a subsequent PR.

Positron Release Notes

New Features

• N/A

Bug Fixes

• N/A