Grammar and Docs consistency fixes

[ruizmaa]

This is the main issue to fix the mismatches between the grammar and the documentation

Sub-issues

ID	Repo	Summary
A1	dpmcore	Relax grammar so not works as a unary prefix operator without mandatory parentheses, matching VTL
A2	dpmcore	Extend sub parser rule to accept multiple substitutions: SUB subAssignment (COMMA subAssignment)*.
A3	dpm-xml-docs	Document all three accepted RHS forms for sub in §11.4.2: literal, select, and itemReference.
A4	dpmcore	Allow operationRef as head of cellAddress with row/col/sheet args.
A5	dpmcore	Remove PRECONDITION_ELEMENT as a separate token and collapse it into the existing varRef rule (which already uses the v_ prefix). Remove the preconditionElem alternative from selectOperand.
A6	dpm-xml-docs	Rewrite §3.1.1 with separate syntax forms per prefix: {tTable, args*}, {vVariable} (no args), {oOperation[, args*]}, {v_Precondition}.
A7	dpm-xml-docs	Document PERCENT_LITERAL in §2.4.2, including semantics (is 25% stored as 0.25 or as a distinct type?).
A8	dpm-xml-docs	Add a sentence to §2.4.2 or §4 clarifying the relationship between "" and null.
A9	dpm-xml-docs	Amend §2.4.1 to state that DPM-XL keywords are lowercase.
A10	dpmcore	Add an ESCAPED_IDENTIFIER token (`) to the lexer.
A11	dpm-xml-docs	Fix §10.1.2: replace scal <str> with a time_period_code enumeration (A | S | Q | M | W | D).
A12	dpmcore	Relax time_shift numberPeriods from INTEGER_LITERAL to expression. This requires upgrading the ANTLR parser engine from 4.9.3 to 4.10+, but due to security is not possible, so the parser should be cahnged to c++.
A13/A14	dpmcore	Restrict rename clause so that only property codes are accepted on both sides. Standard key components (r, c, s) are no longer valid rename targets.
B2	dpmcore + dpm-xml-docs	Remove unused TIME_INTERVAL_LITERAL and TIME_PERIOD_LITERAL tokens from the grammar. Update §2.4.2 to reflect that only DATE_LITERAL exists.
B3	dpm-xml-docs	Add an operator precedence table (12 levels, derived from grammar rule ordering) to docs/operators/01-introduction.md. Use VTL's precedence table as a reference.
B6/C2	dpm-xml-docs	Replace - with a comparison operator (e.g. = or >) in the example at §4.3 of 04-null-treatment.md.
C1	dpm-xml-docs	Fix §6.4.1.6: not behaviour references 22-binary-operators; should reference 21-unary-operators.
C4	dpm-xml-docs	Add at least one worked example to §8.2.7 (nvl) and §8.3.7 (filter), both of which currently have an empty Examples section.
C5	dpm-xml-docs	Replace the Last updated: [Current Date] placeholder in dpm_xl_style_guide.md:189 with the actual date.
C6	dpm-xml-docs	Rename fallback_value to default_value in the §3.1.1 syntax block to match the rest of the document and the grammar.
C7	dpm-xml-docs	Fix typo Component2 → Component in §3.2.7 Examples 6 and 9.
C8	dpm-xml-docs	Verify whether c0255 in §3.2.7 Example 8 is correct for EBA COREP table C 08.01.a (columns are in the 0010–0270 range).