docs(Guide): add a why-spec page, and stop three places underselling it

[DerManoMann]

Overview

The spec pipeline has a guide, a reference, an extension-points page and a tab on the homepage
snippet. It has nowhere that answers why anyone would move to it.

Three of the places a reader forms a first impression argue against it by accident. The
homepage info box's operative sentence is "the mode needs to be set to spec" — a chore,
above the snippet, before any reason to care. The modes table offers spec as "Best for: New
projects", which tells every existing project not to look. The README bullet is a list of
internals ending in "version-aware compilers".

The new page leads with the thing classic attributes cannot do at all: spec attributes are
ordinary objects, so a document can be built from values and compiled with nothing to scan —
which is what an integration whose API surface is computed rather than declared needs. The
snippet showing it is executed by the test suite against a committed expectation, so the page
cannot drift from what the library does. The other sections are the mistakes the pipeline
reports instead of absorbing, output versions as compiler targets, and what a Specification
already exposes.

The modes table loses its "Best for" row rather than getting a better answer: the row above it
already says what each mode reads, which is the axis that actually decides the question.

ROADMAP.md also stops predicting on a named third party's behalf. The bridge has nothing to
scan for when an integration builds annotation objects directly rather than scanning for them,
so the v7 exception is now stated as that mechanism — which is the reason, and stays true of
anything shaped that way.

Changes

• Add docs/guide/why-spec.md, with a sidebar entry above "Processing Modes"
• Add docs/snippets/guide/why-spec/value_objects.php and its expected output, executed by a
  new WhySpecSnippetTest
• Rewrite the homepage spec info box to say what the attributes are and link the new page
• Drop the "Best for" row from the modes table
• Replace the README's spec bullet with a reason and a link
• State the v7 bridge exception in ROADMAP.md as a mechanism rather than a named project